Stream Temperature Relationships to Forest Harvest in Western Washington1

Abstract: We compared summer stream temperature patterns in 40 small forested watersheds in the Hoh and Clearwater basins in the western Olympic Peninsula, Washington, to examine correlations between previous riparian and basin‐wide timber harvest activity and stream temperatures. Seven watersheds were unharvested, while the remaining 33 had between 25% and 100% of the total basin harvested, mostly within the last 40 years. Mean daily maximum temperatures were significantly different between the harvested and unharvested basins, averaging 14.5°C and 12.1°C, respectively. Diurnal fluctuations between harvested and unharvested basins were also significantly different, averaging 1.7°C and 0.9°C, respectively. Total basin harvest was correlated with average daily maximum temperature (r² = 0.39), as was total riparian harvest (r² = 0.32). The amount of recently clear‐cut riparian forest (<20 year) within 600 m upstream of our monitoring sites ranged from 0% to 100% and was not correlated to increased stream temperatures. We used Akaike’s Information Criteria (AIC) analysis to assess whether other physical variables could explain some of the observed variation in stream temperature. We found that variables related to elevation, slope, aspect, and geology explain between 5% and 14% more of the variability relative to the variability explained by percent of basin harvested (BasHarv), and that the BasHarv was consistently a better predictor than the amount of riparian forest harvested. While the BasHarv is in all of the models that perform well, the AIC analysis shows that there are many models with two variables that perform about the same and therefore it would be difficult to choose one as the best model. We conclude that adding additional variables to the model does not change the basic findings that there is a relatively strong relationship between maximum daily stream temperatures and the total amount of harvest in a basin, and strong, but slightly weaker relationship between maximum daily stream temperatures and the total riparian harvest in a basin. Seventeen of the 40 streams exceeded the Washington State Department of Ecology’s (DOE) temperature criterion for waters defined as “core salmon and trout habitat” (class AA waters). The DOE temperature criterion for class AA waters is any seven‐day average of daily maximum temperatures in excess of 16°C. The probability of a stream exceeding the water quality standard increased with timber harvest activity. All unharvested sites and five of six sites that had 25‐50% harvest met DOEs water quality standard. In contrast, only nine of eighteen sites with 50‐75% harvest and two of nine sites with >75% harvest met DOEs water quality standard. Many streams with extensive canopy closure, as estimated by the age of riparian trees, still had higher temperatures and greater diurnal fluctuations than the unharvested basins. This suggests that the impact of past forest harvest activities on stream temperatures cannot be entirely mitigated through the reestablishment of riparian buffers.     

Measuring the Impact of Urbanization on Channel Widths Using Historic Aerial Photographs and Modern Surveys1

Abstract: Land use in a watershed is commonly held to exert a strong influence on trunk channel form and process. Land use changes act over human time‐scales, which are short enough to measure effects on channels directly using historic aerial photographs. We show that high‐resolution topographic surveys for the channels of paired watersheds in the Lehigh Valley, Pennsylvania, are comparable, but have channel widths that have changed dramatically in the past five decades. The two watersheds, Little Lehigh Creek and Sacony Creek, are similar in most aspects except in their respective amount of urban land use. Aerial photographs of the urbanized Little Lehigh Creek show that a majority of the measured widths (67 of 85) were statistically wider in 1999 than in 1947. In contrast, the measured widths from the agricultural Sacony Creek are more evenly distributed among those that widened (18), narrowed (28), and those that were statistically unchanged (6) from 1946 to 1999. From 1946 to 1999 the only section of Sacony Creek that widened was that reach downstream of the only sizable urban area in the watershed. The current land use in Sacony Creek watershed resembles that of 1946, while the Little Lehigh Creek watershed has more than tripled its urban area. These data, in concert with other recent hydrologic data from the watersheds suggest that the increase in urban area‐generated peak discharges is the mechanism behind the widening that occurred in the Little Lehigh Creek. These wider channels can affect water quality, aquatic habitat, suspended sediment loads, and river esthetics.     

Critical Evaluation of How the Rosgen Classification and Associated “Natural Channel Design” Methods Fail to Integrate and Quantify Fluvial Processes and Channel Response1

Abstract: Over the past 10 years the Rosgen classification system and its associated methods of “natural channel design” have become synonymous to some with the term “stream restoration” and the science of fluvial geomorphology. Since the mid 1990s, this classification approach has become widely adopted by governmental agencies, particularly those funding restoration projects. The purposes of this article are to present a critical review, highlight inconsistencies and identify technical problems of Rosgen’s “natural channel design” approach to stream restoration. This paper’s primary thesis is that alluvial streams are open systems that adjust to altered inputs of energy and materials, and that a form‐based system largely ignores this critical component. Problems with the use of the classification are encountered with identifying bankfull dimensions, particularly in incising channels and with the mixing of bed and bank sediment into a single population. Its use for engineering design and restoration may be flawed by ignoring some processes governed by force and resistance, and the imbalance between sediment supply and transporting power in unstable systems. An example of how C5 channels composed of different bank sediments adjust differently and to different equilibrium morphologies in response to an identical disturbance is shown. This contradicts the fundamental underpinning of “natural channel design” and the “reference‐reach approach.” The Rosgen classification is probably best applied as a communication tool to describe channel form but, in combination with “natural channel design” techniques, are not diagnostic of how to mitigate channel instability or predict equilibrium morphologies. For this, physically based, mechanistic approaches that rely on quantifying the driving and resisting forces that control active processes and ultimate channel morphology are better suited as the physics of erosion, transport, and deposition are the same regardless of the hydro‐physiographic province or stream type because of the uniformity of physical laws.     

Legacy Effects of Colonial Millponds on Floodplain Sedimentation,Bank Erosion,and Channel Morphology,Mid‐Atlantic,USA1

Abstract: Many rivers and streams of the Mid‐Atlantic Region, United States (U.S.) have been altered by postcolonial floodplain sedimentation (legacy sediment) associated with numerous milldams. Little Conestoga Creek, Pennsylvania, a tributary to the Susquehanna River and the Chesapeake Bay, is one of these streams. Floodplain sedimentation rates, bank erosion rates, and channel morphology were measured annually during 2004‐2007 at five sites along a 28‐km length of Little Conestoga Creek with nine colonial era milldams (one dam was still in place in 2007). This study was part of a larger cooperative effort to quantify floodplain sedimentation, bank erosion, and channel morphology in a high sediment yielding region of the Chesapeake Bay watershed. Data from the five sites were used to estimate the annual volume and mass of sediment stored on the floodplain and eroded from the banks for 14 segments along the 28‐km length of creek. A bank and floodplain reach based sediment budget (sediment budget) was constructed for the 28 km by summing the net volume of sediment deposited and eroded from each segment. Mean floodplain sedimentation rates for Little Conestoga Creek were variable, with erosion at one upstream site (?5 mm/year) to deposition at the other four sites (highest = 11 mm/year) despite over a meter of floodplain aggradation from postcolonial sedimentation. Mean bank erosion rates range between 29 and 163 mm/year among the five sites. Bank height increased 1 m for every 10.6 m of channel width, from upstream to downstream (R² = 0.79, p < 0.0001) resulting in progressively lowered hydraulic connectivity between the channel and the floodplain. Floodplain sedimentation and bank erosion rates also appear to be affected by the proximity of the segments to one existing milldam, which promotes deposition upstream and scouring downstream. The floodplain and bank along the 28‐km reach produced a net mean sediment loss of 5,634 Mg/year for 2004‐2007, indicating that bank erosion was exceeding floodplain sedimentation. In particular, the three segments between the existing dam and the confluence with the Conestoga River (32% of the studied reach) account for 97% of the measured net sediment budget. Future research directed at understanding channel equilibria should facilitate efforts to reduce the sediment impacts of dam removal and legacy sediment.     

Influence of Small Dams on Downstream Channel Characteristics in Pennsylvania and Maryland: Implications for the Long‐Term Geomorphic Effects of Dam Removal1

Abstract: We evaluate the effects of small dams (11 of 15 sites less than 4 m high) on downstream channels at 15 sites in Maryland and Pennsylvania by using a reach upstream of the reservoir at each site to represent the downstream reach before dam construction. A semi‐quantitative geomorphic characterization demonstrates that upstream reaches occupy similar geomorphic settings as downstream reaches. Survey data indicate that dams have had no measurable influence on the water surface slope, width, and the percentages of exposed bedrock or boulders on the streambed. The median grain diameter (D₅₀) is increased slightly by dam construction, but D₅₀ remains within the pebble size class. The percentage of sand and silt and clay on the bed averages about 35% before dam construction, but typically decreases to around 20% after dam construction. The presence of the dam has therefore only influenced the fraction of finer‐grained sediment on the bed, and has not caused other measurable changes in fluvial morphology. The absence of measurable geomorphic change from dam impacts is explicable given the extent of geologic control at these study sites. We speculate that potential changes that could have been induced by dam construction have been resisted by inerodible bedrock, relatively immobile boulders, well‐vegetated and cohesive banks, and low rates of bed material supply and transport. If the dams of our study are removed, we argue that long‐term changes (those that remain after a period of transient adjustment) will be limited to increases in the percentage of sand and silt and clay on the bed. Thus, dam removal in streams similar to those of our study area should not result in significant long‐term geomorphic changes.     

Modeling Stream Channel Characteristics From Drainage‐Enforced DEMs in Puget Sound,Washington, USA1

Abstract: Mapping stream channels and their geomorphic attributes is an important step in many watershed research and management projects. Often insufficient field data exist to map hydromorphologic attributes across entire drainage basins, necessitating the application of hydrologic modeling tools to digital elevation models (DEMs) via a geographic information system (GIS). In this article, we demonstrate methods for deriving synthetic stream networks via GIS across large and diverse basins using drainage‐enforced DEMs, along with techniques for estimating channel widths and gradient on the reach scale. The two‐step drainage enforcement method we used produced synthetic stream networks that displayed a high degree of positional accuracy relative to the input streams. The accuracies of our estimated channel parameters were assessed with field data, and predictions of bankfull width, wetted width and gradient were strongly correlated with measured values (r² = 0.92, r² = 0.95, r² = 0.88, respectively). Classification accuracies of binned channel attributes were also high. Our methodology allows for the relatively rapid mapping of stream channels and associated morphological attributes across large geographic areas. Although initially developed to provide salmon recovery planners with important salmon habitat information, we suggest these methodologies are relevant to a variety of research and management questions.     

Water‐Yield Reduction After Afforestation and Related Processes in the Semiarid Liupan Mountains,Northwest China1

Abstract: The increase of coverage of forest/vegetation is imperative to improve the environment in dry‐land areas of China, especially for protecting soil against serious erosion and sandstorms. However, inherent severe water shortages, drought stresses, and increasing water use competition greatly restrict the reforestation. Notably, the water‐yield reduction after afforestation generates intense debate about the correct approach to afforestation and forest management in dry‐land areas. However, most studies on water‐yield reduction of forests have been at catchment scales, and there are few studies of the response of total evapotranspiration (ET) and its partitioning to vegetation structure change. This motivates us to learn the linkage between hydrological processes and vegetation structure in slope ecosystems. Therefore, an ecohydrological study was carried out by measuring the individual items of water balance on sloping plots covered by different vegetation types in the semiarid Liupan Mountains of northwest China. The ratio of precipitation consumed as ET was about 60% for grassland, 93% for shrubs, and >95% for forestland. Thus, the water yield was very low, site‐specific, and sensitive to vegetation change. Conversion of grassland to forest decreased the annual water yield from slope by 50‐100 mm. In certain periods, the plantations at lower slopes even consumed the runon from upper slopes. Reducing the density of forests and shrubs by thinning was not an efficient approach to minimize water use. Leaf area index was a better indicator than plant density to relate ET to vegetation structure and to evaluate the soil water carrying capacity for vegetation (i.e., the maximum amount of vegetation that can be supported by the available soil water for an extended time). Selecting proper vegetation types and plant species, based on site soil water condition, may be more effective than the forest density regulation to minimize water‐yield reduction by vegetation coverage increase and notably by reforestation. Finally, the focuses in future research to improve the forest‐water relations in dry‐land areas are recommended as follows: vegetation growth dynamics driven by environment especially water conditions, coupling of ecological and hydrological processes, further development of distributed ecohydrological models, quantitative relation of eco‐water quota of ecosystems with vegetation structures, multi‐scaled evaluation of soil water carrying capacity for vegetation, and the development of widely applicable decision support tools.     

Comparison of Macroinvertebrate‐Derived Stream Quality Metrics Between Snag and Riffle Habitats1

Abstract: We compared benthic macroinvertebrate assemblage structure at snag and riffle habitats in 43 Wisconsin streams across a range of watershed urbanization using a variety of stream quality metrics. Discriminant analysis indicated that dominant taxa at riffles and snags differed; Hydropsychid caddisflies (Hydropsyche betteni and Cheumatopsyche spp.) and elmid beetles (Optioservus spp. and Stenemlis spp.) typified riffles, whereas isopods (Asellus intermedius) and amphipods (Hyalella azteca and Gammarus pseudolimnaeus) predominated in snags. Analysis of covariance indicated that samples from snag and riffle habitats differed significantly in their response to the urbanization gradient for the Hilsenhoff biotic index (BI), Shannon’s diversity index, and percent of filterers, shredders, and pollution intolerant Ephemeroptera, Plecoptera, and Trichoptera (EPT) at each stream site (p ≤ 0.10). These differences suggest that although macroinvertebrate assemblages present in either habitat type are sensitive to detecting the effects of urbanization, metrics derived from different habitats should not be intermixed when assessing stream quality through biomonitoring. This can be a limitation to resource managers who wish to compare water quality among streams where the same habitat type is not available at all stream locations, or where a specific habitat type (i.e., a riffle) is required to determine a metric value (i.e., BI). To account for differences in stream quality at sites lacking riffle habitat, snag‐derived metric values can be adjusted based on those obtained from riffles that have been exposed to the same level of urbanization. Comparison of nonlinear regression equations that related stream quality metric values from the two habitat types to percent watershed urbanization indicated that snag habitats had on average 30.2 fewer percent EPT individuals, a lower diversity index value than riffles, and a BI value of 0.29 greater than riffles.     

Urban Impacts on Streams are Scale‐Dependent With Nonlinear Influences on Their Physical and Biotic Recovery in Vermont,United States1

Fitzgerald, Evan P., William B. Bowden, Samuel P. Parker, and Michael L. Kline, 2012. Urban Impacts on Streams Are Scale‐Dependent with Nonlinear Influences on Their Physical and Biotic Recovery in Vermont, United States. Journal of the American Water Resources Association (JAWRA) 48(4): 679‐697. DOI: 10.1111/j.1752‐1688.2012.00639.x Abstract: The physical and biological conditions of stream reaches in 16 watersheds within the Lake Champlain Basin of Vermont, United States, were assessed and analyzed for a response to total impervious area (TIA) at multiple spatial scales. Natural gradients (e.g., channel slope) and human impacts to channel boundary conditions (e.g., bank armoring) were considered to ensure a robust test of the Impervious Cover Model for upslope TIA. The response of geomorphic stability and sensitive macroinvertebrates to TIA was nonlinear and significant (p < 0.001), decreasing rapidly at 5% TIA. The effect of urbanization on stream condition was shown to interact significantly with drainage area and channel slope using the analysis of covariance (ANCOVA) (p < 0.05). Hydraulic geometry regressions for urban and rural watersheds and ANCOVA were used to describe a significant watershed scale‐dependent response of channel width to urbanization (p = 0.001). The analysis of macroinvertebrate data from reaches in different stages of channel evolution indicated that stable reaches supported greater richness of pollution intolerant species (p < 0.001) and overall taxa richness (p < 0.01) than unstable reaches, and that biotic integrity improves as channels regain stability during their evolution into a state of quasi‐equilibrium. We conclude that macroinvertebrate communities can respond positively to channel evolution processes leading to natural channel restabilization.     

Fine‐Sediment Loadings to Lake Tahoe1

Abstract: Over the past 35 years, a trend of decreasing water clarity has been documented in Lake Tahoe, attributable in part to the delivery of fine‐grained sediments emanating from upland and channel sources. The overall objective of the research reported here was to determine the amount of fine sediment delivered to Lake Tahoe from each of the 63 contributing watersheds. The research described in this report used combinations of field‐based observations of channel and bank stability with measured and simulated data on fine‐sediment loadings to estimate fine‐sediment loadings from unmonitored basins throughout the Lake Tahoe Basin. Loadings were expressed in the conventional format of mass per unit time but also in the number of particles finer than 20 μm, the latter for future use in a lake‐clarity model. The greatest contributors of fine sediment happened to be those with measured data, not requiring extrapolation. In descending order, they are as follows: Upper Truckee River [1,010 tonnes per year (T/year)], Blackwood Creek (846 T/year), Trout Creek (462 T/year), and Ward Creek (412 T/year). Summing estimated values from the contributing watersheds provided an average, annual estimate of fine‐sediment (<0.063 mm) loadings to the lake of 5,206 T/year. A total of 7.79E + 19 particles in the 5‐20 μm fraction were calculated to enter Lake Tahoe in an average year with the Upper Truckee River accounting for almost 25% of the total. Contributions from Blackwood, Ward, Trout, and Third creeks account for another 23% of these very fine particles. Thus, these five streams making up about 40% of the basin area, account for almost 50% of all fine‐sediment loadings to the lake. Contribution of fine sediment from streambank erosion were estimated by developing empirical relations between measured or simulated bank‐erosion rates with a field‐based measure of the extent of bank instability along given streams. An average, annual fine‐sediment loading from streambank erosion of 1,305 T/year was calculated. This represents about 25% of the average, annual fine‐sediment load delivered to the lake from all sources. The two largest contributors, the Upper Truckee River (639 T/year) and Blackwood Creek (431 T/year), account for slightly more than 80% of all fines emanating from streambanks, representing about 20% of the fine sediment delivered to Lake Tahoe from all sources. Extrapolations of fine‐sediment loadings to the unmonitored watersheds are based on documented empirical relations, yet contain a significant amount of uncertainty. Except for those values derived directly from measured data, reported results should be considered as estimates.     

Estimating the Frequency and Extent of Overbank Flow of a Baseflow‐Dominated Stream Flowing through a Wetland

River floodplains provide critical habitat for a wide range of animal and plant species and reduce phosphorus and nitrogen loads in streams. It has been observed that baseflow‐dominated streams flowing through wetlands are commonly at or near bankfull and overflow their banks much more frequently than other streams. However, there is very little published quantitative support for this observation. The study focuses on a 1‐km reach of Black Earth Creek, a stream in the Midwestern United States (U.S.). We used one‐dimensional hydraulic modeling to estimate bankfull discharge at evenly spaced stream cross sections, and two‐dimensional modeling to quantitate the extent of wetland inundation as a function of discharge. We then used historical streamflow data from two U.S. Geological Survey gaging stations to quantitate the frequency of wetland inundation. For the with‐sediment case, the frequency of overbank conditions at the 38 cross sections in the wetland ranged from 3 to 85 days per year and averaged 43 days per year. Ten percent of the wetland was inundated for an average of 35 days per year. For the without‐sediment case, the frequency of overbank conditions ranged from 2.6 to 48 days per year and averaged 14 days per year. Also, 10% of the wetland was inundated for an average of 25 days per year. These unusually high rates of floodplain inundation are likely due in part to the very low stream gradient and shallow depths of overbank flow.     

Comparison of Stream Invertebrate Response Models for Bioassessment Metrics1

Waite, Ian R., Jonathan G. Kennen, Jason T. May, Larry R. Brown, Thomas F. Cuffney, Kimberly A. Jones, and James L. Orlando, 2012. Comparison of Stream Invertebrate Response Models for Bioassessment Metrics. Journal of the American Water Resources Association (JAWRA) 48(3): 570-583. DOI: 10.1111/j.1752-1688.2011.00632.x Abstract: We aggregated invertebrate data from various sources to assemble data for modeling in two ecoregions in Oregon and one in California. Our goal was to compare the performance of models developed using multiple linear regression (MLR) techniques with models developed using three relatively new techniques: classification and regression trees (CART), random forest (RF), and boosted regression trees (BRT). We used tolerance of taxa based on richness (RICHTOL) and ratio of observed to expected taxa (O/E) as response variables and land use/land cover as explanatory variables. Responses were generally linear; therefore, there was little improvement to the MLR models when compared to models using CART and RF. In general, the four modeling techniques (MLR, CART, RF, and BRT) consistently selected the same primary explanatory variables for each region. However, results from the BRT models showed significant improvement over the MLR models for each region; increases in R² from 0.09 to 0.20. The O/E metric that was derived from models specifically calibrated for Oregon consistently had lower R² values than RICHTOL for the two regions tested. Modeled O/E R² values were between 0.06 and 0.10 lower for each of the four modeling methods applied in the Willamette Valley and were between 0.19 and 0.36 points lower for the Blue Mountains. As a result, BRT models may indeed represent a good alternative to MLR for modeling species distribution relative to environmental variables.     

Effects of Land‐Use and Land‐Cover Change on Evapotranspiration and Water Yield in China During 1900‐20001

Abstract: China has experienced a rapid land‐use/cover change (LUCC) during the 20th Century, and this process is expected to continue in the future. How LUCC has affected water resources across China, however, remains uncertain due to the complexity of LUCC‐water interactions. In this study, we used an integrated Dynamic Land Ecosystem Model (DLEM) in conjunction with spatial data of LUCC to estimate the LUCC effects on the magnitude, spatial and temporal variations of evapotranspiration (ET), runoff, and water yield across China. Through comparisons of DLEM results with other model simulations, field observations, and river discharge data, we found that DLEM model can adequately catch the spatial and seasonal patterns of hydrological processes. Our simulation results demonstrate that LUCC led to substantial changes in ET, runoff, and water yield in most of the China’s river basins during the 20th Century. The temporal and spatial patterns varied significantly across China. The largest change occurred during the second half century when almost all of the river basins had a decreasing trend in ET and an increasing trend in water yield and runoff, in contrast to the inclinations of ET and declinations of water yield in major river basins, such as Pearl river basin, Yangtze river basin, and Yellow river basin during the first half century. The increased water yield and runoff indicated alleviated water deficiency in China in the late 20th Century, but the increased peak flow might make the runoff difficult to be held by reservoirs. The continuously increasing ET and decreasing water yield in Continental river basin, Southwest river basin, and Songhua and Liaohe river basin implied regional water deficiency. Our study in China indicates that deforestation averagely increased ET by 138 mm/year but decreased water yield by the same amount and that reforestation averagely decreased ET by 422 mm/year since most of deforested land was converted to paddy land or irrigated cropland. In China, cropland‐related land transformation is the dominant anthropogenic force affecting water resources during the 20th Century. On national average, cropland expansion was estimated to increase ET by 182 mm/year while cropland abandonment decreased ET by 379 mm/year. Our simulation results indicate that urban sprawl generally decreased ET and increased water yield. Cropland managements (fertilization and irrigation) significantly increased ET by 98 mm/year. To better understand LUCC effects on China’s water resources, it is needed to take into account the interactions of LUCC with other environmental changes such as climate and atmospheric composition.     

Climate and Land Use Effects on Hydrologic Processes in a Primarily Rain‐Fed,Agricultural Watershed

Anticipating changes in hydrologic variables is essential for making socioeconomic water resource decisions. This study aims to assess the potential impact of land use and climate change on the hydrologic processes of a primarily rain‐fed, agriculturally based watershed in Missouri. A detailed evaluation was performed using the Soil and Water Assessment Tool for the near future (2020–2039) and mid‐century (2040–2059). Land use scenarios were mapped using the Conversion of Land Use and its Effects model. Ensemble results, based on 19 climate models, indicated a temperature increase of about 1.0°C in near future and 2.0°C in mid‐century. Combined climate and land use change scenarios showed distinct annual and seasonal hydrologic variations. Annual precipitation was projected to increase from 6% to 7%, which resulted in 14% more spring days with soil water content equal to or exceeding field capacity in mid‐century. However, summer precipitation was projected to decrease, a critical factor for crop growth. Higher temperatures led to increased potential evapotranspiration during the growing season. Combined with changes in precipitation patterns, this resulted in an increased need for irrigation by 38 mm representing a 10% increase in total irrigation water use. Analysis from multiple land use scenarios indicated converting agriculture to forest land can potentially mitigate the effects of climate change on streamflow, thus ensuring future water availability.     

POST‐HARVEST RIPARIAN BUFFER RESPONSE: IMPLICATIONS FOR WOOD RECRUITMENT MODELING AND BUFFER DESIGN1

Despite the importance of riparian buffers in providing aquatic functions to forested streams, few studies have sought to capture key differences in ecological and geomorphic processes between buffered sites and forested conditions. This study examines post‐harvest buffer conditions from 20 randomly selected harvest sites within a managed tree farm in the Cascade Mountains of western Washington. Post‐harvest wind derived treefall rates in buffers up to three years post‐harvest averaged 268 trees/km/year, 26 times greater than competition‐induced mortality rate estimates. Treefall rates and stem breakage were strongly tied to tree species and relatively unaffected by stream direction. Observed treefall direction is strongly biased toward the channel, irrespective of channel or buffer orientation. Fall direction bias can deliver significantly more wood recruitment relative to randomly directed treefall, suggesting that models that utilize the random fall assumption will significantly underpredict recruitment. A simple estimate of post‐harvest wood recruitment from buffers can be obtained from species specific treefall and breakage rates, combined with bias corrected recruitment probability as a function of source distance from the channel. Post‐harvest wind effects may reduce the standing density of trees enough to significantly reduce or eliminate competition mortality and thus indirectly alter bank erosion rates, resulting in substantially different wood recruitment dynamics from buffers as compared to unmanaged forests.     

Spatial Variability of Pool-Tail Fines in Mountain Gravel-Bed Stream Affects Grid-Count Results1

Bunte, Kristin, John P. Potyondy, Kurt W. Swingle, and Steven R. Abt, 2012. Spatial Variability of Pool-Tail Fines in Mountain Gravel-Bed Stream Affects Grid-Count Results. Journal of the American Water Resources Association (JAWRA) 48(3): 530-545. DOI: 10.1111/j.1752-1688.2011.00629.x Abstract: Fine sediment (<2 and <6 mm) particles underlying a 49-intersection grid placed on a streambed at 25, 50, and 75% of the wetted pool-tail width are commonly counted to assess the status and trend of aquatic ecosystems or to monitor changes in the supply of fines in mountain gravel-bed streams. However, results vary even when crews perform nearly identical procedures. This study hypothesized that spatial variability of pool-tail fines affects grid-count results and that a sampling scheme can be optimized for precision and accuracy. Grid counts taken at seven evenly spaced locations across the wetted width of 10 pool tails in a pool-riffle study stream indicated a bankward fining trend with secondary peaks of fines within the stream center. Sampling locations close to the waterlines harbored more than twice as many fines as central locations. Most of the five grid-count schemes derived from the seven sampled locations produced significantly different results. Compared with sampling at all seven locations, schemes that focus near waterlines overpredicted fines, while those that focus on the center underpredicted them. Variability of fines among pool tails was the highest within a broad band along the waterlines; hence, focusing sampling there yielded the most variable results. The scheme sampling at 25, 50, and 75% of the wetted width had the lowest precision and moderate accuracy. Accuracy and precision of grid-count results can be greatly improved by sampling at seven even-spaced locations across the pool tail.     

Impacts of Land‐Cover Change on Suspended Sediment Transport in Two Agricultural Watersheds1

Schilling, Keith E., Thomas M. Isenhart, Jason A. Palmer, Calvin F. Wolter, and Jean Spooner, 2011. Impacts of Land‐Cover Change on Suspended Sediment Transport in Two Agricultural Watersheds. Journal of the American Water Resources Association (JAWRA) 47(4):672‐686. DOI: 10.1111/j.1752‐1688.2011.00533.x Abstract: Suspended sediment is a major water quality problem, yet few monitoring studies have been of sufficient scale and duration to assess the effectiveness of land‐use change or conservation practice implementation at a watershed scale. Daily discharge and suspended sediment export from two 5,000‐ha watersheds in central Iowa were monitored over a 10‐year period (water years 1996‐2005). In Walnut Creek watershed, a large portion of land was converted from row crop to native prairie, whereas in Squaw Creek land use remained predominantly row crop agriculture. Suspended sediment loads were similar in both watersheds, exhibiting flashy behavior typical of incised channels. Modeling suggested that expected total soil erosion in Walnut Creek should have been reduced 46% relative to Squaw Creek due to changes in land use, yet measured suspended sediment loads showed no significant differences. Stream mapping indicated that Walnut Creek had three times more eroding streambank lengths than did Squaw Creek suggesting that streambank erosion dominated sediment sources in Walnut Creek and sheet and rill sources dominated sediment sources in Squaw Creek. Our results demonstrate that an accounting of all sources of sediment erosion and delivery is needed to characterize sediment reductions in watershed projects combined with long‐term, intensive monitoring and modeling to account for possible lag times in the manifestation of the benefits of conservation practices on water quality.     

Understanding the Factors That Influence Headwater Stream Flows in Response to Storm Events1

Stanfield, Les W. and Don A. Jackson, 2011. Understanding the Factors That Influence Headwater Stream Flows in Response to Storm Events. Journal of the American Water Resources Association (JAWRA) 1‐22. DOI: 10.1111/j.1752‐1688.2010.00518.x Abstract: Headwater drainage features (first‐ to second‐order streams) are the capillaries of the landscape that, among other things, moderate the timing and volumes of water available to the riparian and aquatic ecosystems. How these features respond to summer rainfall is poorly understood. We studied how geology and an index of land use/land cover influenced peak flows following rainfall events in 110 headwater stream sites that were studied over a four‐month period during a drought year. Highest peak flows were observed in the most urbanized catchments and in poorly drained soils, but specific responses were variable depending on both geology and land disturbance. Redundancy analysis indicated that both surficial geology and land disturbance were important factors influencing peak flows under drought conditions. We conclude that responses of these headwater streams to individual storms during drought conditions are unpredictable from data collected using our methods, but increased peak flows were associated with increased urban and agricultural development, but mitigated by surficial geology. These findings demonstrate the challenges to accurately predict flow conditions in headwater streams during periods of extreme weather that concurrently have the greatest potential effect on biota. The combination of these challenges and importance of such events indicates the need to develop new approaches to study and manage these resources.     

Multi‐Decadal Remote‐Sensing Analysis of Irrigated Areas in the Lower Rio Grande Valley,New Mexico

A time series of estimates of irrigated area was developed for the Lower Rio Grande valley (LRG) in New Mexico from the 1970s to present day. The objective of the project was to develop an independent, accurate, and scientifically justifiable evaluation of irrigated area in the region for the period spanning from the mid‐1970s to the present. These area estimates were used in support of groundwater modeling of the LRG region, as well as for other analyses. This study used a remote‐sensing‐based methodology to evaluate overall irrigated area within the LRG. We applied a methodology that involved the normalization of vegetation indices derived from satellite imagery to get a more accurate estimation of irrigated area across multiple time periods and multiple Landsat platforms. The normalization allows more accurate evaluation of vegetation index data that span several decades. An accuracy assessment of the methodology and results from this study was performed using field‐collected crop data from the 2008 growing season. The comparisons with field data indicate that the accuracy of the remote‐sensing‐based estimates of historical irrigated area is very good, with rates of false positives (areas identified as irrigated that are not truly irrigated) of only about 4%, and rates of false negatives (areas identified as not irrigated that are truly irrigated) in the range of 0.6‐2.0%.