A Method to Consider Whether Dams Mitigate Climate Change Effects on Stream Temperatures

This article provides a method for examining mesoscale water quality objectives downstream of dams with anticipated climate change using a multimodel approach. Coldwater habitat for species such as trout and salmon has been reduced by water regulation, dam building, and land use change that alter stream temperatures. Climate change is an additional threat. Changing hydroclimatic conditions will likely impact water temperatures below dams and affect downstream ecology. We model reservoir thermal dynamics and release operations (assuming that operations remain unchanged through time) of hypothetical reservoirs of different sizes, elevations, and latitudes with climate‐forced inflow hydrologies to examine the potential to manage water temperatures for coldwater habitat. All models are one dimensional and operate on a weekly timestep. Results are presented as water temperature change from the historical time period and indicate that reservoirs release water that is cooler than upstream conditions, although the absolute temperatures of reaches below dams warm with climate change. Stream temperatures are sensitive to changes in reservoir volume, elevation, and latitude. Our approach is presented as a proof of concept study to evaluate reservoir regulation effects on stream temperatures and coldwater habitat with climate change.     

Regulated flushing in a gravel-bed river for channel habitat maintenance: A Trinity River fisheries case study

The operation of Trinity and Lewiston Dams on the Trinity River in northern California in the United States, combined with severe watershed erosion, has jeopardized the existence of prime salmonid fisheries. Extreme streamflow depletion and stream sedimentation below Lewiston have resulted in heavy accumulation of coarse sediment on riffle gravel and filling of streambed pools, causing the destruction of spawning, nursery, and overwintering habitat for prized chinook salmon (Salmo gairdnerii) and steelhead trout (Oncorhynchus tschawytscha). Proposals to restore and maintain the degraded habitat include controlled one-time remedial peak flows or annual maintenance peak flows designed to flush the spawning gravel and scour the banks, deltas, and pools. The criteria for effective channel restoration or maintenance by streambed flushing and scouring are examined here, as well as the mechanics involved.The liabilities of releasing mammoth scouring-flushing flows approximating the magnitude that preceded reservoir construction make this option unviable. The resulting damage to fish habitat established under the postproject streamflow regime, as well as damage to human settlements in the floodplain, would be unacceptable, as would the opportunity costs to hydroelectric and irrigation water users. The technical feasibility of annual maintenance flushing flows depends upon associated mechanical and structural measures, particularly instream maintenance dredging of deep pools and construction of a sediment control dam on a tributary where watershed erosion is extreme. The cost effectiveness of a sediment dam with a limited useful economic life, combined with perpetual maintenance dredging, is questionable.     

Dam Design can Impede Adaptive Management of Environmental Flows: A Case Study from the Opuha Dam, New Zealand

The Opuha Dam was designed for water storage, hydropower, and to augment summer low flows. Following its commissioning in 1999, algal blooms (dominated first by Phormidium and later Didymosphenia geminata) downstream of the dam were attributed to the reduced frequency and magnitude of high-flow events. In this study, we used a 20-year monitoring dataset to quantify changes associated with the dam. We also studied the effectiveness of flushing flows to remove periphyton from the river bed. Following the completion of the dam, daily maximum flows downstream have exceeded 100 m³ s^?1 only three times; two of these floods exceeded the pre-dam mean annual flood of 203 m³ s^?1 (compared to 19 times >100 m³ s^?1 and 6 times >203 m³ s^?1 in the 8 years of record before the dam). Other changes downstream included increases in water temperature, bed armoring, frequency of algal blooms, and changes to the aquatic invertebrate community. Seven experimental flushing flows resulted in limited periphyton reductions. Flood wave attenuation, bed armoring, and a shortage of surface sand and gravel, likely limited the effectiveness of these moderate floods. Floods similar to pre-dam levels may be effective for control of periphyton downstream; however, flushing flows of that magnitude are not possible with the existing dam infrastructure. These results highlight the need for dams to be planned and built with the capacity to provide the natural range of flows for adaptive management, particularly high flows.     

ASSESSING FLUSHING-FLOW REQUIREMENTS FOR BROWN TROUT SPAWNING GRAVELS IN STEEP STREAMS1

Flushing flows are re1eses from dams designed to remove fine sediment from downstream spawning habitat. We evaluated flushing flows on reaches proposed for hydroelectric diversions on seven streams in the eastern Sierra Nevada, California, with wild populations of brown trout (Salmo trutta). The stream reaches are steep (average map slopes range from 7 to 17 percent), are dominated by boulder cascades, and afford few opportunities for gravel deposition. Methods for estimating flushing flows from flow records, developed from studies in other localities, produced widely differing results when applied to the study streams, probably reflecting differences in the hydrologic and geomorphic characteristics of the streams on which the methods were developed. Tracer gravel experiments demonstrated that all sampled gravels were washed out by the flows of 1986, a wet year. Size analyses of gravel samples and hydraulic data from field surveys were used in tractive-force calculations in an attempt to specify the flow required to flush the gravels. However, these calculations produced some unrealistic results because the flows were nonuniform in the study reaches. This suggests that the tractive-force approach may not be generally applicable to small, steep streams where nonuniform flow conditions prevail.     

A modeling assessment of the thermal regime for an urban sport fishery

Water temperature is almost certainly a limiting factor in the maintenance of a self-sustaining rainbow trout (Oncorhynchus mykiss, formerlySalmo gairdneri) and brown trout (Salmo trutta) fishery in the lower reaches of the Cache la Poudre River near Fort Collins, Colorado, USA. Irrigation diversions dewater portions of the river, but cold reservoir releases moderate water temperatures during some periods. The US Fish and Wildlife Service’s Stream Network Temperature Model (SNTEMP) was applied to a 31-km segment of the river using readily available stream geometry and hydrological and meteorological data. The calibrated model produced satisfactory water temperature predictions (R ²=0.88,P<0.001, N=49) for a 62-day summer period. It was used to evaluate a variety of flow and nonflow alternatives to keep water temperatures below 23.3°C for the trout. Supplemental flows or reduced diversions of 3 m³/sec would be needed to maintain suitable summer temperatures throughout most of the study area. Such flows would be especially beneficial during weekends when current irrigation patterns reduce flows. The model indicated that increasing the riparian shade would result in little improvement in water temperatures but that decreasing the stream width would result in significant temperature reductions. Introduction of a more thermally tolerant redband trout (Oncorhynchus sp.), or smallmouth bass (Micropterus dolomieui) might prove beneficial to the fishery. Construction of deep pools for thermal refugia might also be helpful.     

Effects of discharge fluctuation and the addition of fine sediment on stream fish and macroinvertebrates below a water-filtration facility

A small, coastal stream in the San Francisco Bay area of California, USA, received the discharges from a drinking-water filtration plant. Two types of discharges were present. Discharges from filter backwashing were 3–4 times base stream flow, occurred 10–60 times per day, contained fine sediments, and each lasted about 10 min. The other discharge was a large, steady flow of relatively sediment-free water from occasional overflow of the delivery aqueduct which generally lasted several hours a day.Samples of invertebrates from natural substrates had significantly fewer taxa and lower density at the two stations below the backwash than at the two above. However, when stable artificial substrates were used, there were no significant differences among all four stations. The aqueduct apparently had no effect because the. invertebrate community at the station upstream of the backwash but downstream of the aqueduct was statistically similar to the station above the aqueduct. To test for acute toxicity, we exposed additional artificial substrates to short-term simulated backwash conditions. These exposures had no effect on invertebrate density or drift. Three-spine stickleback (Gasterosteus aculeatus) populations were also significantly reduced at the two downstream stations and were made up mostly of larger, adult fish. Prickly sculpins (Cottus asper), restricted to the most downstream station, were emaciated and had poor growth, probably as a result of scarce benthic food organisms. Artificial redds with eggs of rainbow trout (Salmo gairdneri) had significantly lower survival at two stations below the plant backwash (30.7% and 41.8%) than at the one above it (61.4%). Hatchery rainbow trout held in cages below the treatment plant from 7 to 37 days survived and continued to feed.Thus, the major effect of the water treatment plant on fish and invertebrates probably was not from acute toxicity in the discharges or the occasionally large discharge of clean water from the aqueduct, but was from the fluctuating backwash flows containing fine sediment that displaced small fish downstream and created unstable benthic substrates for invertebrates.The filter plant that we studied is a direct-feed type (that is, no sedimentation before filtration). These generally require greater frequencies of backwashing than do conventional plants and may therefore have greater biological impacts. Direct-feed plants are becoming increasingly popular throughout the world, for the most part because they are cheaper to build and operate. But if the associated biological problems are mitigated, then the cost savings of direct-feed compared to conventional plants may be lost.     

Effect of reservoir flushing on downstream river water quality

The effect of short-term reservoir flushing on downstream water quality in the Geum River, Korea was studied using field experiments and computer simulations. The reservoir release was increased from 30 to 200 m(3)/s within 6 h for the purpose of this experiment. The flushing discharge decreased the concentrations of soluble nitrogen and phosphorus species considerably, but the experimental results revealed a negative impact on organic forms of nutrients and biochemical oxygen demand (BOD). A dynamic river water quality model was applied to simulate the river hydraulics and water quality variations during the event. The model showed very good performance in predicting the travel time of flushing flow and the variations of dissolved forms of nitrogen and phosphorus constituents. However, it revealed a limited capability in simulating organic forms of nutrients and BOD because it does not consider the re-suspension mechanism of these constituents from sediment during the wave front passage.     

Fish Assemblage Responses to Water Withdrawals and Water Supply Reservoirs in Piedmont Streams

Understanding effects of flow alteration on stream biota is essential to developing ecologically sustainable water supply strategies. We evaluated effects of altering flows via surface water withdrawals and instream reservoirs on stream fish assemblages, and compared effects with other hypothesized drivers of species richness and assemblage composition. We sampled fishes during three years in 28 streams used for municipal water supply in the Piedmont region of Georgia, U.S.A. Study sites had permitted average withdrawal rates that ranged from < 0.05 to > 13 times the stream’s seven-day, ten-year recurrence low flow (7Q10), and were located directly downstream either from a water supply reservoir or from a withdrawal taken from an unimpounded stream. Ordination analysis of catch data showed a shift in assemblage composition at reservoir sites corresponding to dominance by habitat generalist species. Richness of fluvial specialists averaged about 3 fewer species downstream from reservoirs, and also declined as permitted withdrawal rate increased above about 0.5 to one 7Q10-equivalent of water. Reservoir presence and withdrawal rate, along with drainage area, accounted for 70% of the among-site variance in fluvial specialist richness and were better predictor variables than percent of the catchment in urban land use or average streambed sediment size. Increasing withdrawal rate also increased the odds that a site’s Index of Biotic Integrity score fell below a regulatory threshold indicating biological impairment. Estimates of reservoir and withdrawal effects on stream biota could be used in predictive landscape models to support adaptive water supply planning intended to meet societal needs while conserving biological resources.     

DECISION FRAMEWORK FOR SEDIMENT CONTROL IN MUDDY CREEK WATERSHED1

ABSTRACT: The tailwater of Bridgewater Dam, below Lake James, North Carolina, is a designated trout stream. It has environmental attributes for a good cold water fishery with the exception of high suspended sediments. Muddy Creek, a tributary about 1.5 km downstream of the dam, is a major source of sediments. The Muddy Creek Watershed Restoration Initiative was established to develop and implement a sediment control plan. The Watershed Analysis Risk Management Framework was applied to simulate soil erosion and sedimentation and to help determine appropriate action. The simulated sediment concentrations of the river were comparable to observed data from November 1994 to November 2001. For the base condition, the sediment load was 135,000 kg/d from surface erosion and 1,300,000 kg/d from bank erosion. Increasing the buffer strip from existing 50 to 80 percent to 100 percent of stream segments would only reduce surface erosion to 70,400 kg/d with little change in sediment concentrations. Eliminating riverbank erosion would reduce the sediment load from 920,000 to 87,700 kg/d. The bank stabilization project would not only lower suspended sediment concentrations for Muddy Creek, but also reduce the lake sediment accumulation in the downstream Lake Rhodhiss by approximately 13 percent.     

Suspended-Sediment Concentration Regimes for Two Biological Reference Streams in Middle Tennessee1

Diehl, Timothy H. and William J. Wolfe, 2010. Suspended-Sediment Concentration Regimes for Two Biological Reference Streams in Middle Tennessee. Journal of the American Water Resources Association (JAWRA) 46(4): 824-837. DOI: 10.1111/j.1752-1688.2010.00460.x Abstract: Temporal patterns of suspended-sediment concentration (SSC) duration and frequency (SSC regimes) were characterized and compared with biological impairment thresholds for two headwater streams in the Western Highland Rim of Tennessee. The SSC regimes were plotted as curves showing concentrations and durations of the annual longest and tenth-longest SSC excursions above 18 concentrations for water years 2005-2008 in Copperas Branch and water years 2006 and 2008 in Kelley Creek. Both streams have fish communities remarkably diverse for their small drainage basin areas (420 and 565 ha, respectively), and represent biological reference conditions with respect to SSC. SSC-regime curves were similar for the two sites across water years. The measured SSC regimes reached or exceeded published experimentally based SSC impairment thresholds and plotted below a proposed long-term SSC reference regime for the Interior Plateau ecoregion (Ecoregion 71), suggesting that neither the experimentally based thresholds nor the proposed SSC reference regime adequately reflect the relation between SSC and biological impairment for Western Highland Rim headwater streams. The SSC regimes of the two study streams were similar to the estimated SSC regime of an unimpaired East Tennessee trout stream. Additional field studies are needed to describe SSC regimes in streams of varying basin scale, level of impairment, and region.     

A Method for Spatially Explicit Representation of Sub-watershed Sediment Yield,Southern California,USA

We present here a method to integrate geologic, topographic, and land-cover data in a geographic information system to provide a fine-scale, spatially explicit prediction of sediment yield to support management applications. The method is fundamentally qualitative but can be quantified using preexisting sediment-yield data, where available, to verify predictions using other independent data sets. In the 674-km² Sespe Creek watershed of southern California, 30 unique “geomorphic landscape units” (GLUs, defined by relatively homogenous areas of geology, hillslope gradient, and land cover) provide a framework for discriminating relative rates of sediment yield across this landscape. Field observations define three broad groupings of GLUs that are well-associated with types, relative magnitudes, and rates of erosion processes. These relative rates were then quantified using sediment-removal data from nearby debris basins, which allow relatively low-precision but robust calculations of both local and whole-watershed sediment yields, based on the key assumption that minimal sediment storage throughout most of the watershed supports near-equivalency of long-term rates of hillslope sediment production and watershed sediment yield. The accuracy of these calculations can be independently assessed using geologically inferred uplift rates and integrated suspended sediment measurements from mainstem Sespe Creek, which indicate watershed-averaged erosion rates between about 0.6–1.0 mm year^?1 and corresponding sediment yields of about 2 × 10³ t km^?2 year^?1. A spatially explicit representation of sediment production is particularly useful in a region where wildfires, rapid urban development, and the downstream delivery of upstream sediment loads are critical drivers of both geomorphic processes and land-use management.     

ESTIMATED STREAM FLOW REQUIREMENTS FOR FISHES OF THE WASHITA RIVER BELOW FOSS RESERVOIR,WESTERN OKLAHOMA1

Velocity and depth preference curves for juvenile and adult red shiners (Notropis lutrensis) were developed for each season of the year. The incremental methodology, developed by the U.S. Fish and Wildlife Service for quantifying the amount of physical habitat available for fish, was applied as a basis for recommending minimum stream flows for life stages of red shiners and channel catfish (Ictalurus punctatus) in the Washita River below Foss Reservoir. The minimum stream flow required to support populations of red shiners and channel catfish was estimated to be 0.60 m³/s during all months except April and May, during which flushing flows of 3.0 m³/s were recommended. Under drought conditions a contingency flow of 0.18 m³/s was recommended for August through March, a flushing flow of 2.4 m³/s for April and May, and a flow of 0.30 m³/s for channel catfish spawning in June and July. Standing crop of adult red shiners per weighted usable area averaged 2.73 g/m² (27 kg/ha) for two sites on the Washita River. A positive relation between standing crop and weighted usable area was suggested. Use of the incremental methodology on the study reach below Foss Reservoir required 96 person-hours of labor, excluding training, travel, and developing preference curves, and a total cost of $1,762, of which $1,200 was for the purchase of equipment.     

Stream Nitrogen Sources Apportionment and Pollution Control Scheme Development in an Agricultural Watershed in Eastern China

A modeling system that couples a land-use-based export coefficient model, a stream nutrient transport equation, and Bayesian statistics was developed for stream nitrogen source apportionment. It divides a watershed into several sub-catchments, and then considers the major land-use categories as stream nitrogen sources in each sub-catchment. The runoff depth and stream water depth are considered as the major factors influencing delivery of nitrogen from land to downstream stream node within each sub-catchment. The nitrogen sources and delivery processes are lumped into several constant parameters that were calibrated using Bayesian statistics from commonly available stream monitoring and land-use datasets. This modeling system was successfully applied to total nitrogen (TN) pollution control scheme development for the ChangLe River watershed containing six sub-catchments and four land-use categories. The temporal (across months and years) and spatial (across sub-catchments and land-use categories) variability of nonpoint source (NPS) TN export to stream channels and delivery to the watershed outlet were assessed. After adjustment for in-stream TN retention, the time periods and watershed areas with disproportionately high-TN contributions to the stream were identified. Aimed at a target stream TN level of 2 mg L^?1, a quantitative TN pollution control scheme was further developed to determine which sub-catchments, which land-use categories in a sub-catchment, which time periods, and how large of NPS TN export reduction were required. This modeling system provides a powerful tool for stream nitrogen source apportionment and pollution control scheme development at the watershed scale and has only limited data requirements.     

SETTING AN EFFECTIVE TMDL: SEDIMENT LOADING AND EFFECTS OF SUSPENDED SEDIMENT ON FISR1

ABSTRACT: The Agricultural Drainage and Pesticide Transport model was used to examine the relationship between fish and suspended sediment in the context of a proposed total maximum daily load (TMDL) in two agricultural watersheds in Minnesota. During a 50‐year simulation, Wells Creek, a third‐order cold water stream, had an estimated 1,164 events (i.e., one or more consecutive days of estimated sediment loading) and the Chippewa River, a fourth‐order warm water stream, had 906 events of measurable suspended sediment. Sublethal thresholds were exceeded for 970 events and lethal levels for 194 events for brown trout in Wells Creek, whereas adult nonsalmonids would have experienced sublethal levels for 923 events and lethal levels for 241 events. Sublethal levels were exceeded for 756 events and lethal thresholds were exceeded for 150 events in the Chippewa River. Nonsalmonids would have experienced 15 events of mortality between 0 and 20 percent in Wells Creek. In the Chippewa River, there were 35 events of mortality between 0 and 20 percent and one event in which mortality could have exceeded 20 percent. The Minnesota Pollution Control Agency has proposed listing stream reaches as being impaired for turbidity at 25 NTU, which is approximately 46 mg suspended sediment/1. We estimated that 46 mg/1 would be exceeded approximately 30 days in a year (d/yr) in both systems. A TMDL of 46 mg SS/1 may be too high to ensure that stream fishes are not negatively affected by suspended sediment. We recommend that an indicator incorporating the duration of exposure be applied.     

EFFECTS OF WATERSHED BEST MANAGEMENT PRACTICES ON HABITAT AND FISH IN WISCONSIN STREAMS1

ABSTRACT: We evaluated the effectiveness of watershed‐scale implementations of best‐management practices (BMPs) for improving habitat and fish attributes in two coldwater stream systems in Wisconsin. We sampled physical habitat, water temperature, and fish communities in multiple paired treatment and reference streams before and after upland (barnyard runoff controls, manure storage, contour plowing, reduced tillage) and riparian (stream bank fencing, sloping, limited rip‐rapping) BMP installation in the treatment subwatersheds. In Spring Creek, BMPs significantly improved overall stream habitat quality, bank stability, instream cover for fish, abundance of cool‐ and coldwater fishes, and abundance of all fishes. Improvements were most pronounced at sites with riparian BMPs. Water temperatures were consistently cold enough to support coldwater fishes such as trout (Salmonidae) and sculpins (Cottidae) even before BMP installation. We observed the first‐time occurrence of naturally reproduced brown trout (Salmo trutta) in Spring Creek, indicating that the stream condition had been improved to be able to partially sustain a trout population. In Eagle Creek and its tributary Joos Creek, limited riparian BMPs led to localized gains in overall habitat quality, bank stability, and water depth. However, because few upland BMPs were installed in the subwatershed there were no improvements in water temperature or the quality of the fish community. Temperatures remained marginal for coldwater fish throughout the study. Our results demonstrate that riparian BMPs can improve habitat conditions in Wisconsin streams, but cannot restore coldwater fish communities if there is insufficient upland BMP implementation. Our approach of studying multiple paired treatment and reference streams before and after BMP implementation proved effective in detecting the response of stream ecosystems to watershed management activities.     

ASSESSMENT USING GIS AND SEDIMENT ROUTING OF THE PROPOSED REMOVAL OF BALLVILLE DAM,SANDUSKY RIVER,OHIO1

ABSTRACT: The proposed removal of Ballville Dam was assessed by (1) using a new Geographic Information Systems (GIS) based method for calculating reservoir sediment storage, (2) evaluating sediment properties and contamination from core data, and (3) assessing downstream impacts from sediment routing calculations. A 1903 (pre‐dam) map was manipulated using GIS to recreate the reservoir bathymetry at time of dam construction and used in combination with a detailed 1993 bathymetric survey to calculate sediment volumes and thickness. Reservoir sediment properties and geochemistry were determined from 14 sediment vibracores. Annual sedimentation rates varied from 1.7 to 4.3 g/cm²/yr based on Cesium‐137 (¹³⁷Cs) and Lead‐210 (²¹⁰Pb) geochronology and dated flood layers. The pore fluid geochemistry (Ba, Co, Cu, Mn) of four cores showed surficial enrichments in Cu, while Co and Mn show secondary peaks within the sediments. GIS calculations showed that a designed channel through the former reservoir able to accommodate the 10 percent Probable Maximum Flood (PMF) would require removing approximately 0.35 million m³ of sediment (27 percent of the reservoir fill), either by dredging at a cost of up to $6.3 million or by releasing fine grained sediment downstream. A sediment routing model was applied for the critical 6 km downstream using four cross sections. The sediment routing model predicts that, for flows exceeding minimum Mean Daily Flow (1924 to 1998 data), greater than 90 percent of this sediment would be transported through downstream reaches into Lake Erie (Sandusky Bay).     

PHOSPHORUS ASSIMILATION IN A STREAM SYSTEM OF THE LAKE OKEECHOBEE BASIN1

ABSTRACT: The ability to predict how streams and wetlands retain phosphorus (P) is critical to the management of watersheds that contribute nutrients to adjacent aquatic systems such as lakes. Field and laboratory experiments were conducted to determine the P assimilatory capacity of a stream (Otter Creek) in the Taylor Creek/Nubbin Slough Basin located north of Lake Okeechobee, Florida. Dominant soils in this basin are sandy Spodosols; landuse is primarily dairy farms and beef cattle pastures. Estimates of P assimilation show that sediments assimilate approximately 5 percent of the P load. Phosphorus assimilation rates in the stream were estimated using first-order relationships based on the total P concentration of the water column as a function of distance from the primary source. This method assumes minimal lateral inputs. Stream lengths required for one turnover in P assimilation were estimated to be in the range of 3–16 km. Laboratory studies using intact sediment cores indicated a P assimilation rate of 0.025 m day^?1, and equilibrium P concentration of 0.16 ± 0.03 mg L^?1 in the water column. Dissolved P concentration gradients in the sediments showed upward flux of P at water column P concentration of <0.16 mg L^?1. Approximately 56–77 percent of the P assimilated in the above-ground vegetation during active growth was released or translocated within six months of senesence, suggesting short-term storage in above-ground vegetation. Bottom sediments and recalcitrant detrital plant tissue provide for long-term P assimilation in the creek. Although stream sediments have the potential to adsorb P, high flow rate and low contact period between water and sediment limits this process.     

Seasonality of Soil Erosion Under Mediterranean Conditions at the Alqueva Dam Watershed

The Alqueva reservoir created the largest artificial lake of Western Europe in 2010. Since then, the region has faced challenges due to land-use changes that may increase the risk of erosion and shorten the lifetime of the reservoir, increasing the need to promote land management sustainability. This paper investigates the aspect of seasonality of soil erosion using a comprehensive methodology that integrates the Revised Universal Soil Loss Equation (RUSLE) approach, geographic information systems, geostatistics, and remote-sensing. An experimental agro-silvo pastoral area (typical land-use) was used for the RUSLE factors update. The study confirmed the effect of seasonality on soil erosion rates under Mediterranean conditions. The highest rainfall erosivity values occurred during the autumn season (433.6 MJ mm ha^?1 h^?1), when vegetation cover is reduced after the long dry season. As a result, the autumn season showed the highest predicted erosion (9.9 t ha^?1), contributing 65 % of the total annual erosion. The predicted soil erosion for winter was low (1.1 t ha^?1) despite the high rainfall erosivity during that season (196.6 MJ mm ha^?1 h^?1). The predicted annual soil loss was 15.1 t ha^?1, and the sediment amount delivery was 4,314 × 10³ kg. Knowledge of seasonal variation would be essential to outline sustainable land management practices. This model will be integrated with World Overview of Conservation Approaches and Technologies methods to support decision-making in that watershed, and it will involve collaboration with both local people and governmental institutions.     

BIOGEOCIIEMISTRY OF AN OLD-GROWTH FORESTED WATERSHED,OLYMPIC NATIONAL PARK,WASHINGTON1

ABSTRACT: The biogeochemistry of a coastal old-growth forested watershed in Olympic National Park, Washington, was examined. Objectives were to determine: (1) concentrations of major cations and anions and dissolved organic C (DOC) in precipitation, throughfall, stemflow, soil solution and the stream; (2) nutrient input/output budgets; and (3) nutrient retention mechanisms in the watershed. Stemilow was more acidic (pH 4.0–4.5) than throughfall (pH 5.1) and precipitation (pH 5.3). Organic acids were important contributors to acidity in throughfall and stemflow and tree species influenced pH. Soil solution pH averaged 6.2 at 40 cm depth. Stream pH was higher (7.6). Sodium (54.0 μeq L-¹) and Cl (57.6 μeq L^?1) were the dominant ions in precipitation, reflecting the close proximity to the ocean. Throughfall and stemflow were generally enriched in cations, especially K. Cation concentrations in soil solutions were generally less than those in stemilow. Ion concentrations increased in the stream. Dominant ions were Ca (759.7 μeq L^?1), Na (174.4 μeq L^?1), HCO₃ (592.0 μeq L^?1), and SO₄ (331.5 μeq L^?1) with seasonal peaks in the fall. Bedrock weathering strongly influenced stream chemistry. Highest average NO₃ concentrations were in the stream (5.2 μeq L^?1) with seasonal peaks in the fall and lowest concentrations in the growing season. Nitrogen losses were similar to inputs; annual inputs were 4.8 kg/ha (not including fixation) and stream losses were 7.1 kg/ha. Despite the age and successional status of the forest, plant uptake is an important N retention mechanism in this watershed.