SUSPENDED SEDIMENT DYNAMICS IN SMALL FOREST STREAMS OF THE PACIFIC NORTHWEST1

This paper reviews suspended sediment sources and transport in small forest streams in the Pacific Northwest region of North America, particularly in relation to riparian management. Mass movements, reading and yarding practices, and burning can increase the supply of suspended sediment. Sediment yields recovered to pre‐harvest levels within one to six years in several paired catchment studies. However, delayed mass movements related to roads and harvesting may produce elevated suspended sediment yield one or more decades after logging. There is mixed evidence for the role of streamside tree throw in riparian buffers in supplying sediment to streams. Harvesting within the riparian zone may not increase suspended sediment yield if near stream soils are not disturbed. Key knowledge gaps relate to the relative roles of increased transport capacity versus sediment supply, the dynamics of fine sediment penetration into bed sediments, and the effects of forest harvesting on suspended sediment at different scales. Future research should involve nested catchments to examine suspended sediment response to forest practices at multiple spatial scales, in combination with process‐based field studies.     

Applications of Turbidity Monitoring to Forest Management in California

Many California streams have been adversely affected by sedimentation caused by historic and current land uses, including timber harvesting. The impacts of timber harvesting and logging transportation systems on erosion and sediment delivery can be directly measured, modeled, or inferred from water quality measurements. California regulatory agencies, researchers, and land owners have adopted turbidity monitoring to determine effects of forest management practices on suspended sediment loads and water quality at watershed, project, and site scales. Watershed-scale trends in sediment discharge and responses to current forest practices may be estimated from data collected at automated sampling stations that measure turbidity, stream flow, suspended sediment concentrations, and other water quality parameters. Future results from these studies will provide a basis for assessing the effectiveness of modern forest practice regulations in protecting water quality. At the project scale, manual sampling of water column turbidity during high stream flow events within and downstream from active timber harvest plans can identify emerging sediment sources. Remedial actions can then be taken by managers to prevent or mitigate water quality impacts. At the site scale, manual turbidity sampling during storms or high stream flow events at sites located upstream and downstream from new, upgraded, or decommissioned stream crossings has proven to be a valuable way to determine whether measures taken to prevent post-construction erosion and sediment production are effective. Turbidity monitoring at the project and site scales is therefore an important tool for adaptive management. Uncertainty regarding the effects of current forest practices must be resolved through watershed-scale experiments. In the short term, this uncertainty will stimulate increased use of project and site-scale monitoring.     

DISCHARGE AND SUSPENDED SEDIMENT PATTERNS OF AN INTERMITTENT COLD DESERT STREAM1

Sage Creek in south‐central Wyoming is listed as impaired by the U.S. Environmental Protection Agency (USEPA) due to its sediment contribution to the North Platte River. Despite the magnitude of sediment impacts on streams, little research has been conducted to characterize patterns of sediment transport or to model suspended sediment concentration in many arid western U.S. streams. This study examined the relationship between stream discharge and suspended sediment concentration near the Sage Creek and North Platte River confluence from 1998 through 2003. The objectives were to determine patterns of stream discharge and suspended sediment concentration, produce a sediment prediction model, and compare sediment concentrations for the six‐year period. Stream discharge and suspended sediment transport responded rapidly to convective storms and spring runoff events. During the study period, events exceeding 0.23 m³/s accounted for 92 percent of the sediment load, which is believed to originate from erodible headwater uplands. Further analysis of these data indicates that time series modeling is superior to simple linear regression in predicting sediment concentration. Significant increases in suspended sediment concentration occurred in all years except 2003. This analysis suggests that a six‐year monitoring record was insufficient to factor out impacts from climate, geology, and historical sediment storage.     

FOREST HARVESTING AND STREAMWATER INORGANIC CHEMISTRY IN WESTERN NORTH AMERICA: A REVIEW1

The solution chemistry of forested streams primarily in western North America is explained by considering the major factors that influence this chemistry — geological weathering; atmospheric precipitation and climate; precipitation acidity; terrestrial biological processes; physical/chemical reactions in the soil; and physical, chemical, and biological processes within streams. Due to the complexity of all these processes and their varying importance for different chemicals, stream water chemistry has exhibited considerable geographic and temporal variation and is difficult to model accurately. The impacts of forest harvesting on stream water chemistry were reviewed by considering the effects of harvesting on each of the important factors controlling this chemistry, as well as other factors influencing these impacts ‐ extent of the watershed harvested, presence of buffer strips between streams and harvested areas, nature of post‐harvesting site preparation, revegetation rate following harvesting, pre‐harvesting soil fertility, and soil buffering capacity. These effects have sometimes reinforced one another but have sometimes been counterbalancing or slight so that harvesting impacts on stream water chemistry have been highly variable. Eight major knowledge gaps were identified, two of which — a scarcity of detailed stream chemical budgets and knowledge of longitudinal variation in stream chemistry — relate to undisturbed streams, while the remainder relate to forest harvesting effects.     

Water Quality Signals from Rural Land Use and Exurbanization in a Mountain Landscape: What's Clear and What's Confounded?

In mountainous landscapes with high climatic and geomorphic variability, how do rural land uses and exurbanization alter hydrology and water quality? We evaluated effects of rural land use and exurbanization on streamflows, suspended sediment concentrations and loads, specific conductance, and summer water temperatures in 12 streams and rivers within the Upper Little Tennessee River basin in the southern Appalachian Mountains. Eleven streams featured low levels of development (>61% forest cover) but differed in land use patterning, basin size, annual precipitation, and watershed morphology. One urban stream, located within the largest town in the basin, provided the high development comparative endpoint. Even low levels of rural development and exurbanization were associated with substantial increases in suspended sediment concentrations, sediment loads, and summer stream temperature daily maxima and diurnal variation. Observed summer temperature increases were much larger than would be expected due to global climate change over the next century. Specific conductance was idiosyncratic among the smaller streams. These water quality changes were not accompanied by streamflow changes that were discernible amid the high natural variation in precipitation and geomorphology. The water quality findings suggest the need for applying the best management practices, including riparian buffers, to even low levels of rural development.     

Direct and Indirect Effects of Forest Harvesting on Sediment Yield in Forested Watersheds of the United States

Managed forests generally produce high water quality, but degradation is possible via sedimentation if proper management is not implemented during forest harvesting. To mitigate harvesting effects on total watershed sediment yield, it is necessary to understand all processes that contribute to these effects. Forest harvesting best management practices (BMPs) focus almost exclusively on overland sediment sources, whereas in‐and‐near stream sources go unaddressed although they can contribute substantially to sediment yield. Thus, we propose a new framework to classify forest harvesting effects on stream sediment yield according to their direct and indirect processes. Direct effects are those caused by erosion and sediment delivery to surface water from overland sources (e.g., forest roads). Indirect effects are those caused by a shift in hydrologic processes due to tree removal that accounts for increases in subsurface and surface flows to the stream such that alterations in water quality are not predicated upon overland sediment delivery to the stream, but rather in‐stream processes. Although the direct/indirect distinction is often implicit in forest hydrology studies, we have formalized it as a conceptual model to help identify primary drivers of sediment yield after forest harvesting in different landscapes. Based on a literature review, we identify drivers of these effects in five regions of the United States, discuss current forest management BMPs, and identify research needs.     

Research Gaps Related to Forest Management and Stream Sediment in the United States

Water quality from forested landscapes tends to be very high but can deteriorate during and after silvicultural activities. Practices such as forest harvesting, site preparation, road construction/use, and stream crossings have been shown to contribute sediment, nutrients, and other pollutants to adjacent streams. Although advances in forest management accompanied with Best Management Practices (BMPs) have been very effective at reducing water quality impacts from forest operations, projected increases in demand for forest products may result in unintended environmental degradation. Through a review of the pertinent literature, we identified several research gaps related to water yield, aquatic habitat, sediment source and delivery, and BMP effectiveness that should be addressed for streams in the United States to better understand and address the environmental ramifications of current and future levels of timber production. We explored the current understanding of these topics based on relevant literature and the possible implications of increased demand for forest products in the United States.     

GOLD-MINING EFFECTS ON STREAM HYDROLOGY AND WATER QUALITY,CIRCLE QUADRANGLE,ALASKA1

Effects of placer mining on the hydrology and water quality of several interior Alaska streams were studied as part of a project on the impacts of placer mining on stream ecosystems. Surface and subsurface waters were analyzed in the field for conductivity, pH, temperature, alkalinity, total and calcium hardnesses, iron, copper, manganese, ammonia-N, nitrate-N, nitrite-N, settleable solids, and turbidity. Total, nonfiltrable, and filtrable residues were determined in the laboratory. In the streams placer mining increased turbidity, settleable solids, nonfiltrable and filtrable residues and total iron. Surface and subsurface water levels, as measured in wells driven in the stream beds, were correlated with stream flow. Fine sediment deposited on stream beds in mined drainages reduced the hydraulic contact between the surface and subsurface waters of the stream and caused the piezometric water level to be below the surface water level of the mined streams. This resulted in higher specific conductance and significantly lower dissolved oxygen concentrations in the subsurface waters of mined streams compared to their surface waters. No significant differences were found for any water quality characteristics comparing surface to subsurface waters for the unmined streams.     

EVALUATION OF HYDROLOGIC MODELING TECHNIQUES FOR FOREST LAND MANAGEMENT PLANNING1

ABSTRACT: A growing concern for environmental quality paralleled with increasing demands on our forest resources has prompted the Washington State Department of Natural Resources to evaluate simulation modeling as a technique for analyzing management decisions in terms of their environmental effects. The evaluation focused on a system of integrated models developed at the University of Washington which simulate processes and activities within the forest ecosystem. A major part of the system is a hydrologic model which predicts changes in discharge, stream temperature, and concentrations of suspended sediment and dissolved oxygen based on information generated by other models representing intensive management practices. The evaluation consisted of applying the system to a 72,000 acre tract of forest land, validating the models with two years of discharge and water quality data from a 93,000 acre watershed, and determining the pertinence of hydrologic modeling for management purposes. Results show several potential uses of hydrologic modeling for forest management planning, especially for analyzing the effects of timber harvesting strategies on water quality.     

RIPARIAN MICROCLIMATE AND STREAM TEMPERATURE RESPONSE TO FOREST HARVESTING: A REVIEW1

Forest harvesting can increase solar radiation in the riparian zone as well as wind speed and exposure to air advected from clearings, typically causing increases in summertime air, soil, and stream temperatures and decreases in relative humidity. Stream temperature increases following forest harvesting are primarily controlled by changes in insolation but also depend on stream hydrology and channel morphology. Stream temperatures recovered to pre‐harvest levels within 10 years in many studies but took longer in others. Leaving riparian buffers can decrease the magnitude of stream temperature increases and changes to riparian microclimate, but substantial warming has been observed for streams within both unthinned and partial retention buffers. A range of studies has demonstrated that streams may or may not cool after flowing from clearings into shaded environments, and further research is required in relation to the factors controlling downstream cooling. Further research is also required on riparian microclimate and its responses to harvesting, the influences of surface/subsurface water exchange on stream and bed temperature regimes, biological implications of temperature changes in headwater streams (both on site and downstream), and methods for quantifying shade and its influence on radiation inputs to streams and riparian zones.     

ECOSYSTEM MODELING OF A FORESTED RIVER BASIN1

ABSTRACT: A present concern in decision making processes for forest land use is the environmental effects of land use activities on water, air, and the land itself. Criteria for evaluating the magnitude and detriment of environmental impacts are not definite since it is often difficult to isolate a particular activity as the cause of a particular impact. Instead, interactions between various forest practices must be considered along with their integrated impacts. In order to provide an effective decision tool, the College of Forest Resources, University of Washington, is modeling the forest ecosystem of the Snohomish River Basin located in the Cascade Mountains of western Washington. The project consists of a general system model comprised of subsystem models dealing with product conversion processes, forest production processes, recreation supply processes, wildlife and fisheries supply processes, and the interactions of these processes with water and the atmosphere. The system model is interfaced with a computerized multiple player management game which enables land managers, manufacturing managers, and regulation agency personnel to make management decisions and respond to indications of lack of environmental control. Responses of the hydrologic system to various management decisions are simulated by the water subsystem model. The responses being considered include surface flow quantity and water quality. The model emphasizes the monitoring of non-point as well as point source impacts rather than predicting short-term hydrographs. The significance of impacts vary with land use patterns and the goals of the game player. Therefore, the model has flexible resolution and is able to predict hydrologic conditions for both large and small scale. The water subsystem model responds to management decisions by interpreting the effects of management options selected by game players for 40-acre cells within the Basin. The model then determines which streams are immediately affected, defines the watersheds contributing to these streams, and extracts from a resource data bank the information needed to define model parameters. Using these parameters and precipitation inputs, mean flow discharge on a montly and annual basis is calculated for the impactcd sub watersheds as well as 21 major watersheds of the Basin. Water quality responses predicted for these watersheds include suspended sediment concentration, temperature increases due to stream exposure, dissolved oxygen concentrations, the effects of fertilization on nitrogen content, biocide and herbicide effects, and residues from product conversion processes.     

EFFECTIVENESS OF TIMBER HARVEST PRACTICES FOR CONTROLLING SEDIMENT RELATED WATER QUALITY IMPACTS1

ABSTRACT: Timber harvest best management practices (BMPs) in Washington State were evaluated to determine their effectiveness at achieving water quality standards pertaining to sediment related effects. A weight‐of‐evidence approach was used to determine BMP effectiveness based on assessment of erosion with sediment delivery to streams, physical disturbance of stream channels, and aquatic habitat conditions during the first two years following harvest. Stream buffers were effective at preventing chronic sediment delivery to streams and physical disturbance of stream channels. Practices for ground‐based harvest and cable yarding in the vicinity of small streams without buffers were ineffective or only partially effective at preventing water quality impacts. The primary operational factors influencing BMP effectiveness were: the proximity of ground disturbing activities to streams; presence or absence of designated stream buffers; the use of special timber falling and yarding practices intended to minimize physical disturbance of stream channels; and timing of harvest to occur during snow cover or frozen ground conditions. Important site factors included the density of small streams at harvest sites and the steepness of inner stream valley slopes. Recommendations are given for practices that provide a high confidence of achieving water quality standards by preventing chronic sediment delivery and avoiding direct stream channel disturbance.     

Assessing Riparian Conservation Land Management Practice Impacts on Gully Erosion in Iowa

Well-established perennial vegetation in riparian areas of agricultural lands can stabilize the end points of gullies and reduce their overall erosion. The objective of this study was to investigate the impacts of riparian land management on gully erosion. A field survey documented the number of gullies and cattle access points in riparian forest buffers, grass filters, annual row-cropped fields, pastures in which the cattle were fenced out of the stream, and continuously, rotationally and intensive rotationally grazed pastures in three regions of Iowa. Gully lengths, depths and severely eroding bank areas were measured. Gullies exhibited few significant differences among riparian management practices. The most significant differences were exhibited between conservation and agricultural management practices, an indication that conservation practices could reduce gully erosion. Changes in pasture management from continuous to rotational or intensive rotational grazing showed no reductions in gully erosion. It is important to recognize that more significant differences among riparian management practices were not exhibited because the conservation and alternative grazing practices had recently been established. As gully formation is more impacted by upland than riparian management, gully stabilization might require additional upland conservation practices. The existence of numerous cattle access points in pastures where cattle have full access to the stream also indicates that these could be substantial sources of sediment for streams. Finally, the gully banks were less important sediment contributors to streams than the streambanks. The severely eroding bank areas in streams were six times greater than those in the gullies in the monitored reaches.     

THE SUSPENDED SEDIMENT REGIME OF AN OREGON COAST RANGE STREAM1

ABSTRACT: Armored stream segments may affect the suspended sediment regime of small mountain streams in western Oregon by the release of fine sediments stored in the bed gravels. Sieve analysis of bed materials indicated that at least 30 percent of the suspended sediment yield for the 1975–76 winter had been stored in the streambed. Suspended sediment concentrations during storm-generated runoff were influenced by stream discharge and hydrograph characteristics. Sediment-discharge relations for individual storms were characterized by hysteresis loops. A seasonal flushing of fines was shown by a progressive decrease in the ratio of suspended sediment to stream discharge during the winter runoff period.     

Assessing the Potential Impacts to Riparian Ecosystems Resulting from Hemlock Mortality in Great Smoky Mountains National Park

Hemlock Woolly Adelgid (Adelges tsugae) is spreading across forests in eastern North America, causing mortality of eastern hemlock (Tsuga canadensis [L.] Carr.) and Carolina hemlock (Tsuga caroliniana Engelm.). The loss of hemlock from riparian forests in Great Smoky Mountains National Park (GSMNP) may result in significant physical, chemical, and biological alterations to stream environments. To assess the influence of riparian hemlock stands on stream conditions and estimate possible impacts from hemlock loss in GSMNP, we paired hardwood- and hemlock-dominated streams to examine differences in water temperature, nitrate concentrations, pH, discharge, and available photosynthetic light. We used a Geographic Information System (GIS) to identify stream pairs that were similar in topography, geology, land use, and disturbance history in order to isolate forest type as a variable. Differences between hemlock- and hardwood-dominated streams could not be explained by dominant forest type alone as forest type yields no consistent signal on measured conditions of headwater streams in GSMNP. The variability in the results indicate that other landscape variables, such as the influence of understory Rhododendron species, may exert more control on stream conditions than canopy composition. The results of this study suggest that the replacement of hemlock overstory with hardwood species will have minimal impact on long-term stream conditions, however disturbance during the transition is likely to have significant impacts. Management of riparian forests undergoing hemlock decline should, therefore, focus on facilitating a faster transition to hardwood-dominated stands to minimize long-term effects on water quality.     

LAND USE AND STREAMWATER NITRATE-N DYNAMICS IN AN URBAN-RURAL FRINGE WATERSHED1

ABSTRACT: Agricultural and residential activities are key non-point sources of nitrogen pollution in urban-rural fringe areas. A GIS-based watershed approach was used to compare land use indicators (septic system and animal unit densities), to streamwater nitrate-N in the Salmon River near Vancouver, B.C., Canada. The density of septic systems was used as an indicator of residential development while animal unit density was used as an indicator of the intensity of agricultural activity. Nitrate-nitrogen (nitrate-N) concentrations as high as 7.1 mg·L^?1 were found in the mid-portion of the watershed during the summer months, when streamflow is low and groundwater comprises a large proportion of water in the stream. The major aquifer supplying water to the midsection of the watershed is contaminated with nitrate-N. A comparison of the relationships between septic system and animal unit density and nitrate-N in the upstream to downstream direction provided evidence that both residential and agricultural activities contribute to elevated nitrate-N in the Salmon River mainstem. In contrast, only septic system density corresponded to the pattern of streamwater nitrate-N in Coghlan Creek, the main tributary to the Salmon River.     

Agricultural Land Use Effects on Sediment Loading and Fish Assemblages in Two Minnesota (USA) Watersheds

We examined the relationship between water quality and fish communities within two agricultural areas using a computer simulation model. Our analyses focused on a coolwater stream, Wells Creek in southeastern Minnesota, and a warmwater stream, the Chippewa River in western Minnesota. We used the Agricultural Drainage and Pesticide Transport (ADAPT) model in relation to land use to calculate instream suspended sediment concentrations using estimates of sediment delivery, runoff, baseflow and streambank erosion, and quantified the effects of suspended sediment exposure on fish communities. We predicted the effects of agricultural practices on stream fish communities under several possible land use scenarios, with reference to current conditions. Land use changes led to reductions in sediment loading of up to 84% in Wells Creek and 49% in the Chippewa River. The reduction in sediment loading across scenarios may be directly related to a reduction in runoff by about 35% in both study areas. We found a 98% decrease in lethal concentrations of suspended sediment on fish in Wells Creek with an increase in conservation tillage, riparian buffers, and permanent vegetative cover. However, the effects of suspended sediment did not significantly decrease in the Chippewa River. This difference between study areas was likely due to differences in tolerance to suspended sediment between coolwater and warmwater fish communities and differences in topography, runoff and bank erosion between the two streams. The Minnesota Cooperative Fish and Wildlife Research Unit is jointly sponsored by the US Geological Survey, the University of Minnesota, the Minnesota Department of Natural Resources, and the Wildlife Management Institute.     

Sediment measurement and transport modeling: impact of riparian and filter strip buffers

Well-calibrated models are cost-effective tools to quantify environmental benefits of conservation practices, but lack of data for parameterization and evaluation remains a weakness to modeling. Research was conducted in southwestern Oklahoma within the Cobb Creek subwatershed (CCSW) to develop cost-effective methods to collect stream channel parameterization and evaluation data for modeling in watersheds with sparse data. Specifically, (i) simple stream channel observations obtained by rapid geomorphic assessment (RGA) were used to parameterize the Soil and Water Assessment Tool (SWAT) model stream channel variables before calibrating SWAT for streamflow and sediment, and (ii) average annual reservoir sedimentation rate, measured at the Crowder Lake using the acoustic profiling system (APS), was used to cross-check Crowder Lake sediment accumulation rate simulated by SWAT. Additionally, the calibrated and cross-checked SWAT model was used to simulate impacts of riparian forest buffer (RF) and bermudagrass [ (L.) Pers.] filter strip buffer (BFS) on sediment yield and concentration in the CCSW. The measured average annual sedimentation rate was between 1.7 and 3.5 t ha yr compared with simulated sediment rate of 2.4 t ha yr Application of BFS across cropped fields resulted in a 72% reduction of sediment delivery to the stream, while the RF and the combined RF and BFS reduced the suspended sediment concentration at the CCSW outlet by 68 and 73%, respectively. Effective riparian practices have potential to increase reservoir life. These results indicate promise for using the RGA and APS methods to obtain data to improve water quality simulations in ungauged watersheds.     

Radionuclides as indicators of sediment transport in agricultural watersheds that drain to Lake Erie

An issue in evaluating the success of agricultural management practices is the speed that eroded particles make their way through the downstream waters. In this study at Old Woman Creek (OWC) and Rock Creek (RC), two largely agricultural watersheds in Ohio, the flux of sediment and radionuclides (7Be, 210Pb, and 137Cs) in thunderstorm runoff was examined to better understand transport of eroded agricultural soils. The hydrograph in an agricultural area under no-till was similar in timing, but of lesser magnitude, than the hydrograph from a similar-sized area under conventional tillage. The activities of 210Pb and 7Be are linearly correlated and are higher in suspended sediments derived from no-till subbasins than those derived from conventionally tilled subbasins. A suspended sediment plume, identified by its unique radionuclide signature, was traced through 17 km of OWC stream channel in approximately 13.4 h (0.35 m/s). The downstream exponential decrease of 7Be activities in suspended sediments 3 to 12 h after passage of the sediment plume was used to estimate transport distances of suspended sediment from 2 to 17 km, respectively. Transport distances of suspended sediments were also calculated from wave kinematics and indicate that at OWC suspended sediment transport distances were longer in streams draining areas of no-till (19-26 km) than in the streams draining areas of conventional tillage (6-15 km). Suspended sediments travel 7 to 22 km at RC. The transport distances are long relative to the lengths of the stream channel and indicate that erosion control methods implemented in the watershed should be reflected quickly in downstream waters.     

EFFECTS OF BEST MANAGEMENT PRACTICES ON FOREST STREAMWATER QUALITY IN EASTERN KENTUCKY1

ABSTRACT: Forest land managers are concerned about the effects of logging on soil erosion, streamflow, and water quality and are promoting the use of Best Management Practices (BMPs) to control impacts. To compare the effects of BMP implementation on streamwater quality, two of three small watersheds in Kentucky were harvested in 1983 and 1984, one with BMPs, the other without BMPs. There was no effect of clearcutting on stream temperatures. Streamflow increased by 17.8 cm (123 percent) on the BMP watershed during the first 17 months after cutting and by 20.6 cm (138 percent) on the Non-BMP watershed. Water yields remained significantly elevated compared to the uncut watershed 8 years after harvesting. Suspended sediment flux was 14 and 30 times higher on the BMP and Non-BMP Watersheds, respectively, than on the uncut watershed during treatment, and 4 and 6.5 times higher in the 17 months after treatment was complete. Clearcutting resulted in increased concentrations of nitrate, and other nutrients compared to the uncut watershed, and concentrations were highest on the non-BMP watershed. Recovery of biotic control over nutrient losses occurred within three years of clearcutting. The streamside buffer strip was effective in reducing the impact of clearcutting on water yield and sediment flux.