Trapping efficiencies of cultivated and natural riparian vegetation of northern Laos

In northern Laos, intensification of cultivation on sloping land leads to accelerated erosion processes. Management of riparian land may counteract the negative impacts of higher sediment delivery rates on water quality. This study assessed water and sediment concentration trapping efficiencies of riparian vegetation in northern Laos and the effect of cultivation of riparian land on water quality. Runoff flowing in and out of selected riparian sites was monitored by means of open troughs. In 2005, two native grass, two bamboo, and two banana sites were monitored. In 2006, adjacent to steep banana, bamboo, and native grass sites, three upland rice sites were established and monitored. Water trapping efficiency (WTE) and sediment concentration trapping efficiency (SCTE) were calculated on an event basis; means and 95% confidence intervals (CIs) were estimated with a bootstrapping approach. Confidence intervals were large and overlapping among sites. Seepage conditions severely limited trapping efficiency. Native grass resulted in the highest WTE (95% CI, -0.10 to 0.23), which was not significantly different from zero. Banana resulted in the highest SCTE (95% CI, 0.06-0.40). Bamboo had negative WTE and SCTE. Median outflow runoff from rice sites was nine times the inflow. Median outflow sediment concentration from rice sites was two to five times that of their adjacent sites and two to five times the inflow sediment concentration. Although low-tillage banana plantation may reduce sediment concentration of runoff, cultivation of annual crops in riparian land leads to delivery of turbid runoff into the stream, thus severely affecting stream water quality.     

EFFECTS OF A DAIRY LOAFING LOT-BUFFER STRIP ON STREAM WATER QUALITY1

ABSTRACT: A loafing or sacrifice lot is an area located outside of the free stall barn, where a dairy herd spends several hours per day. Sacrifice lots are usually denuded of vegetation and have high concentrations of manure and urine that can contribute significant amounts of sediment, nutrients, and pathogens to nearby surface waters. In this study, stream water quality impacted by direct runoff from a sacrifice lot was monitored for a period of 20 months. Ambient stream water quality was monitored by grab sampling upstream and downstream of the sacrifice lot. During runoff events, stream water quality downstream of the sacrifice lot was monitored with an automatic sampler. Laboratory analyses were conducted for total suspended solids and nutrients (nitrogen and phosphorus compounds). A grass filter strip (GFS) was installed as a buffer downslope of the sacrifice lot 10 months into the study period. The impact of the buffer strip on the standardized pollutant concentrations and loads was evaluated using the non-parametric Wilcoxon test. The Wilcoxon test indicated that there was no significant difference (α= 0.05) in the standardized yield of sediment and dissolved pollutants before and after the GFS installation, except for phosphate-phosphorus and filtered total phosphorus concentrations, and sediment-bound total phosphorus and total kjeldahl nitrogen loads that decreased significantly. However, load decrease could have been partially caused by the smaller rainfall volumes after the GFS installation as compared to the existing condition.     

Suspended sediment, nitrogen and phosphorus concentrations and exports during storm-events to the Tuross estuary, Australia

This paper presents a process for estimating pollutant loads from water quality data, to improve catchment-scale modelling in the region for resource management purposes. It describes a program to estimate suspended sediment, total and dissolved nitrogen and phosphorus loads to the Tuross estuary from the Tuross River catchment (1810 km(2)) of coastal southeast Australia. Event-based water quality sampling results obtained during storm events in 2005 are presented. Event 1, during July 2005 was the largest storm event in terms of peak flow for 3.5 years. Other events monitored were also in July, November and December 2005. The early July 2005 event had a flow-weighted mean suspended sediment (SS) concentration during the first 4 days of 63 mg L(-1). Of the events monitored, this was unusual as it was preceded by drought and had the largest SS concentrations (peaking at 180 mg L(-1)) during the rising-stage. In contrast, the November event had a much lower flow-weighted SS mean (28 mg L(-1)), even though peak flow magnitudes were similar. The July and November 2005 events had peak flows of 12,360 and 11,330 ML day(-1). Low-cost rising-stage siphon samplers were used to collect samples during the rapidly rising phase of these events. The use of such samplers and consideration of time-lead/lag flow adjustments, quantified using cross-correlation analysis to account for hysteresis effects, were incorporated into the load estimation techniques. The technique is a potentially useful approach for understanding relationships between water quality concentrations and flow for modelling catchment source strengths and transport processes.     

Upland disturbance affects headwater stream nutrients and suspended sediments during baseflow and stormflow

Because catchment characteristics determine sediment and nutrient inputs to streams, upland disturbance can affect stream chemistry. Catchments at the Fort Benning Military Installation (near Columbus, Georgia) experience a range of upland disturbance intensities due to spatial variability in the intensity of military training. We used this disturbance gradient to investigate the effects of upland soil and vegetation disturbance on stream chemistry. During baseflow, mean total suspended sediment (TSS) concentration and mean inorganic suspended sediment (ISS) concentration increased with catchment disturbance intensity (TSS: R2= 0.7, p = 0.005, range = 4.0-10.1 mg L(-1); ISS: R2= 0.71, p = 0.004, range = 2.04-7.3 mg L(-1)); dissolved organic carbon (DOC) concentration (R2= 0.79, p = 0.001, range = 1.5-4.1 mg L(-1)) and soluble reactive phosphorus (SRP) concentration (R2= 0.75, p = 0.008, range = 1.9-6.2 microg L(-1)) decreased with increasing disturbance intensity; and ammonia (NH4+), nitrate (NO3-), and dissolved inorganic nitrogen (DIN) concentrations were unrelated to disturbance intensity. The increase in TSS and ISS during storms was positively correlated with disturbance (R2= 0.78 and 0.78, p = 0.01 and 0.01, respectively); mean maximum change in SRP during storms increased with disturbance (r = 0.7, p = 0.04); and mean maximum change in NO3- during storms was marginally correlated with disturbance (r = 0.58, p = 0.06). Soil characteristics were significant predictors of baseflow DOC, SRP, and Ca2+, but were not correlated with suspended sediment fractions, any nitrogen species, or pH. Despite the largely intact riparian zones of these headwater streams, upland soil and vegetation disturbances had clear effects on stream chemistry during baseflow and stormflow conditions.     

Surface runoff water quality in a managed three zone riparian buffer

Managed riparian forest buffers are an important conservation practice but there are little data on the water quality effects of buffer management. We measured surface runoff volumes and nutrient concentrations and loads in a riparian buffer system consisting of (moving down slope from the field) a grass strip, a managed forest, and an unmanaged forest. The managed forest consisted of sections of clear-cut, thinned, and mature forest. The mature forest had significantly lower flow-weighted concentrations of nitrate, ammonium, total Kjeldahl N (TKN), sediment TKN, total N (nitrate + TKN), dissolved molybdate reactive P (DMRP), total P, and chloride. The average buffer represented the conditions along a stream reach with a buffer system in different stages of growth. Compared with the field output, flow-weighted concentrations of nitrate, ammonium, DMRP, and total P decreased significantly within the buffer and flow-weighted concentrations of TKN, total N, and chloride increased significantly within the buffer. All loads decreased significantly from the field to the middle of the buffer, but most loads increased from the middle of the buffer to the sampling point nearest the stream because surface runoff volume increased near the stream. The largest percentage reduction of the incoming nutrient load (at least 65% for all nutrient forms) took place in the grass buffer zone because of the large decrease (68%) in flow. The average buffer reduced loadings for all nutrient species, from 27% for TKN to 63% for sediment P. The managed forest and grass buffer combined was an effective buffer system.     

Coupling PCSWMM and WASP to Evaluate Green Stormwater Infrastructure Impacts to Storm Sediment Loads in an Urban Watershed

We coupled rainfall–runoff and instream water quality models to evaluate total suspended solids (TSS) in Wissahickon Creek, a mid‐sized urban stream near Philadelphia, Pennsylvania. Using stormwater runoff and instream field data, we calibrated the model at a subdaily scale and focused on storm responses. We demonstrate that treating event mean concentrations as a calibration parameter rather than a fixed input can substantially improve model performance. Urban stormwater TSS concentrations vary widely in time and space and are difficult to represent simply. Suspended and deposited sediment pose independent stressors to stream biota and model results suggest that both currently impair stream health in Wissahickon Creek. Retrofitting existing detention basins to prioritize infiltration reduced instream TSS loads by 20%, suggesting that infiltration mitigates sediment more effectively than detention. Infiltrating stormwater from 30% of the watershed reduced instream TSS loads by 47% and cut the frequency of TSS exceeding 100 mg/L by half. Settled loads and the frequency of high TSS values were reduced by a smaller fraction than suspended loads and duration at high TSS values. A widely distributed network of infiltration‐focused projects is an effective stormwater management strategy to mitigate sediment stress. Coupling rainfall–runoff and water quality models is an important way to integrate watershed‐wide impacts and evaluate how management directly affects urban stream health.     

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.     

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.     

Diffuse pollution loading from urban stormwater runoff in Daejeon city, Korea

In this paper, stormwater runoff from an urban watershed with combined sewer systems located in Daejeon metropolitan city, Korea, was characterized to measure the stormwater runoff discharge rates and pollutant concentrations. The observed averaged event mean concentrations (EMCs) of combined sewer overflows (CSO) were 536.1mg TSS/L, 467.7 mg TCODcr/L, 142.7 mg TBOD/L, 16.5mg TN/L, and 13.5mg TP/L. A detention basin was proposed to reduce CSO, and its essential design elements were discussed. The first flush significantly affected contaminant constituents in the descending order of suspended solid>organics>nutrients. Storage volumes for containing the first flush to improve water quality of the receiving stream can be estimated based on the total suspended solid loading. In this study, detention of the first flush equivalent to 5mm of precipitation could reduce CSO-induced diffuse pollution loading to a receiving water body by up to 80% of the total suspended solid loading.     

Particulate phosphorus transport within stream flow of an agricultural catchment

There is interest in quantifying phosphorus (P) loss from intensively grazed dairy landscapes to identify key pathways and target remediation methods. The Bog Burn drains a dairying catchment in Southland, New Zealand, and has been monitored at fortnightly intervals over a 12-mo period at four sites for suspended sediment (SS), dissolved reactive phosphorus (DRP), and total phosphorus (TP). Time-integrated samplers, deployed at 0.6 median water depth at each site (calculated from previous year's flow data), collected sediment samples, which were analyzed for SS, bioavailable phosphorus (BAP), and TP. Mean concentrations of DRP and TP in stream flow and BAP and TP in sediment were generally highest in summer or autumn (0.043 mg DRP L(-1), 0.160 mg TP L(-1), 173 mg BAP kg(-1), 2228 mg TP kg(-1)) and lowest in winter or spring (0.012 mg DRP L(-1), 0.034 mg TP L(-1), 6 mg BAP kg(-1), 711 mg TP kg(-1)), while loads were highest in winter. Analysis of (137)Cs concentrations in trapped sediment, topsoil, subsoil, and stream bed and bank sediment indicated that trapped sediment was derived from topsoil and entered the stream either through tile drainage or, to a lesser extent, overland flow. Because concentrations of DRP and TP in stream flow are in excess of recommended limits for good water quality (>0.01 mg DRP L(-1), 0.033 mg TP L(-1)), management should focus on the topsoil and specifically on decreasing P loss via tile drainage. This is best achieved by decreasing soil Olsen P concentrations, especially because, on average, Olsen P concentrations in the catchment were above the agronomic optimum.     

BIOECONOMIC ANALYSIS OF SELECTED CONSERVATION PRACTICES ON SOIL EROSION AND FRESHWATER FISHERIES1

ABSTRACT: Farmers can generate environmental benefits (improved water quality and fisheries and wildlife habitat), but they may not be able to quantify them. Furthermore, farmers may reduce their incomes from managing lands to produce these positive externalities but receive little monetary compensation in return. This study simulated the relationship between agricultural practices, water quality, fish responses to suspended sediment and farm income within two small watersheds, one of a cool water stream and one of a warm water stream. Using the Agricultural Drainage and Pesticide Transport (ADAPT) model, this study related best management practices (BMPs) to calculated instream suspended sediment concentrations by estimating sediment delivery, runoff, base flow, and streambank erosion to quantify the effects of suspended sediment exposure on fish communities. By implementing selected BMPs in each watershed, annual net farm income declined $18,000 to $28,000 (1 to 3 percent) from previous levels. “Lethal” fish events from suspended sediments in the cool water watershed decreased by 60 percent as conservation tillage and riparian buffers increased. Despite reducing suspended sediments by 25 percent, BMPs in the warm water watershed did not reduce the negative response of the fisheries. Differences in responses (physical and biological) between watersheds highlight potential gains in economic efficiency by targeting BMPs or by offering performance based “green payments.”     

Fate and efficacy of polyacrylamide applied in furrow irrigation: full-advance and continuous treatments

Polyacrylamide (PAM) is applied to 400000 irrigated hectares annually in the USA to control irrigation-induced erosion, yet the fate of dissolved PAM applied in irrigation water is not well documented. We determined the fate of PAM added to furrow streams under two treatments: Initial-10, 10 mg L(-1) PAM product applied only during the initial hours of the irrigation, and Cont-1, 1.0 mg L(-1) PAM product applied continuously during the entire irrigation. The study measured PAM concentrations in 167-m-long PAM-treated furrow streams and along a 530-m tail ditch that received this runoff. Soil was Portneuf silt loam (coarse-silty, mixed, superactive, mesic Durinodic Xeric Haplocalcid) with 1.5% slope. Samples were taken at three times during the irrigations, both during and after PAM application. Polyacrylamide was adsorbed to soil and removed from solution as the streams traversed the soil-lined channels. The removal rate increased with stream sediment concentration. Stream sediment concentrations were higher when PAM concentrations were <2 mg L(-1) a.i., for early irrigations, and when untreated tributary flows combined with the stream. In these cases, PAM concentration decreased to undetectable levels over the flow lengths used in this study. When inflows contained >6 mg L(-1) PAM a.i., stream sediment concentrations were minimal and PAM concentrations did not change down the furrow, though they decreased to undetectable levels within 0.5 h after application ceased. One percent of applied PAM was lost in tail-ditch runoff. This loss could have been eliminated by treating only the furrow advance or not treating the last two irrigations.     

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.     

Assessing Critical Source Areas in Watersheds for Conservation Buffer Planning and Riparian Restoration

A science-based geographic information system (GIS) approach is presented to target critical source areas in watersheds for conservation buffer placement. Critical source areas are the intersection of hydrologically sensitive areas and pollutant source areas in watersheds. Hydrologically sensitive areas are areas that actively generate runoff in the watershed and are derived using a modified topographic index approach based on variable source area hydrology. Pollutant source areas are the areas in watersheds that are actively and intensively used for such activities as agricultural production. The method is applied to the Neshanic River watershed in Hunterdon County, New Jersey. The capacity of the topographic index in predicting the spatial pattern of runoff generation and the runoff contribution to stream flow in the watershed is evaluated. A simple cost-effectiveness assessment is conducted to compare the conservation buffer placement scenario based on this GIS method to conventional riparian buffer scenarios for placing conservation buffers in agricultural lands in the watershed. The results show that the topographic index reasonably predicts the runoff generation in the watershed. The GIS-based conservation buffer scenario appears to be more cost-effective than the conventional riparian buffer scenarios.     

Transport of particulate matter fractions in urban source area pavement surface runoff

This study used manual full cross-sectional flow discrete sampling and suspended sediment concentration (SSC) methods to gravimetrically characterize noncolloidal hetero-disperse particulate matter (PM). This PM was examined as suspended, settleable, and sediment fractions to assess the distinct transport behavior of each PM fraction throughout each runoff event. Eight runoff events loading an urban paved source area watershed were examined to characterize transport of PM (as SSC) and fractions thereof. An event-based PM mass balance demonstrated recoveries exceeding 90%. With respect to PM transport, two phases were differentiated using a first flush index (m = DeltaM/DeltaV) developed in this study. The m >/= 1 and m < 1 transport phases of the coarser settleable/sediment PM accounted for a higher mass fraction of PM transported during higher flow rates, whereas delivery of the finer suspended fraction became more significant at lower flows. A positive relationship between PM concentration and particle size distribution was found for all events. Event mean concentrations (EMCs) of PM (as SSC) were compared with literature EMCs, sampling methods, and PM analysis method (as total suspended solids [TSS]). Particulate matter study results (as SSC) were higher than many published EMCs (as TSS) (p < 0.05). Differences are attributed to full cross-sectional flow sampling and the use of the the SSC method in contrast to automated sampling combined with TSS methods. Representative characterization of hetero-disperse source area PM is important for water chemistry monitoring, regulatory decisions, best management practice performance and maintenance, and PM inventories in urban systems.     

Rethinking the contribution of drained and undrained grasslands to sediment-related water quality problems

Grass vegetation has been recommended for use in the prevention and control of soil erosion because of its dense sward characteristics and stabilizing effect on the soil. A general assumption is that grassland environments suffer from minimal soil erosion and therefore present little threat to the water quality of surface waters in terms of sediment and sorbed contaminant pollution. Our data question this assumption, reporting results from one hydrological year of observations on a field-experiment monitoring overland flow, drain flow, fluxes of suspended solids, total phosphorus (TP), and molybdate-reactive phosphorus (<0.45 mum) in response to natural rainfall events. During individual rainfall events, 1-ha grassland lysimeters yield up to 15 kg of suspended solids, with concentrations in runoff waters of up to 400 mg L(-1). These concentrations exceed the water quality standards recommended by the European Freshwater Fisheries Directive (25 mg L(-1)) and the USEPA (80 mg L(-1)) and are beyond those reported to have caused chronic effects on freshwater aquatic organisms. Furthermore, TP concentrations in runoff waters from these field lysimeters exceeded 800 mug L(-1). These concentrations are in excess of those reported to cause eutrophication problems in rivers and lakes and contravene the ecoregional nutrient criteria in all of the USA ecoregions. This paper also examines how subsurface drainage, a common agricultural practice in intensively managed grasslands, influences the hydrology and export of sediment and nutrients from grasslands. This dataset suggests that we need to rethink the conceptual understanding of grasslands as non-erosive landscapes. Failure to acknowledge this will result in the noncompliance of surface waters to water quality standards.     

Factors controlling sediment and phosphorus export from two Belgian agricultural catchments

Sediment and total phosphorus (TP) export vary through space and time. This study was conducted to determine the factors controlling sediment and TP export in two agricultural catchments situated in the Belgian Loess Belt. At the outlet of these catchments runoff discharge was continuously measured and suspended sediment samples were taken during rainfall events. Within the catchments vegetation type and cover, soil surface parameters, erosion features, sediment pathways, and rainfall characteristics were monitored. Total P content and sediment characteristics such as clay, organic carbon, and suspended sediment concentration were correlated. Total sediment and TP export differ significantly between the monitored catchments. Much of the difference is due to the occurrence of an extreme event in one catchment and the morphology and spatial organization of land use in the catchments. In one catchment, the direct connection between erosive areas and the catchment outlet by means of a road system contributed to a high sediment delivery ratio (SDR) at the outlet. In the other catchment, the presence of a wide valley in the center of the catchment caused sediment deposition. Vegetation also had an effect on sediment production and deposition. Thus, many factors control sediment and TP export from small agricultural catchments; some of these factors are related to the physical catchment characteristics such as morphology and landscape structure and are (semi)permanent, while others, such as vegetation cover and land use, are time dependent.     

Surface and subsurface phosphorus losses from fertilized pasture systems in Ohio

Phosphorus is an essential plant nutrient and critical to agricultural production, but it is also a problem when excessive amounts enter surface waters. Summer rotational grazing and winter feeding beef pasture systems at two fertility levels (56 and 28 kg available P ha(-1)) were studied to evaluate the P losses from these systems via surface runoff and subsurface flow using eight small (0.3-1.1 ha), instrumented watersheds and spring developments. Runoff events from a 14-yr period (1974-1988) were evaluated to determine the relationships between event size in mm, total dissolved reactive phosphorous (TDRP) concentration, and TDRP transport. Most of the TDRP transported was via surface runoff. There were strong correlations (r2 = 0.45-0.66) between TDRP transport and event size for all watersheds, but no significant (P = 0.05) correlations between TDRP concentration and event size. Flow-weighted average TDRP concentrations from the pasture watersheds for the 14-yr period ranged from 0.64 to 1.85 mg L(-1) with a few individual event concentrations as high as 85.7 mg L(-1). The highest concentrations were in events that occurred soon after P fertilizer application. Average seasonal flow-weighted TDRP concentrations for subsurface flow were < 0.05 mg L(-1). Applying P fertilizer to pastures in response to soil tests should keep TDRP concentrations in subsurface flow at environmentally acceptable levels. Management to reduce runoff and avoidance of P fertilizer application when runoff producing rainfall is anticipated in the next few days will help reduce the surface losses of P.     

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.     

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.