Robert Coats

Nutrient Transport in Surface Runoff from a Subalpine Watershed, Lake Tahoe Basin, California

The watershed of Ward Creek, a tributary to oligotrophic Lake Tahoe in the Sierra Nevada, has been investigated since 1971 with the objective of improving our knowledge of processes of nutrient and sediment release and transport to the lake. Quantitative data on selected stream water parameters were collected for 3 yr (1972-1975) at three stations on Ward Creek, two on the main upper tributaries and one near the stream mouth. Comparative data were collected at a stream mouth station on adjacent Blackwood Creek in the 3rd yr. The parameters were selected on the basis of their significance to eutrophication of Lake Tahoe. Precipitation in a normal year is over 90% snow but annual patterns vary widely and rainfall at any time of year can be highly important in sediment and nutrient transport. Water discharge and the flux of suspended sediments, N03-N, phosphorus, iron and trace metals were dominated by the spring snowmelt runoff from mid-April to mid-June. However, in 1974 heavy fall and summer rains accounted for a large percentage of the annual flux of sediments and nutrients in a total of only 14 d. The spring runoff was characterized by distinct diel water discharge patterns. Similar but not coincident patterns were found to exist for sediments and nutrients, including N03-N, but not for soluble phosphorus. The Ward watershed has 87% the area of Blackwood but discharged proportionately much lower quantities of sediment and nutrients in comparable water yields per hectare in water year 1975. This contrast in fluxes was probably accounted for in the history of greater disturbance by man in Black- wood Canyon. The principal source of suspended sediments in Ward Creek was streambank erosion in the lower reaches of the channel. The dominant form of inorganic nitrogen in Ward was N03-N derived from precipitation, symbiotic nitrogen fixation and nitrification of organic nitrogen in forest soil. Phosphorus and iron were almost entirely in particulate form and thus their periods of flux occurred during high flows and sediment transport. Sediment and nutrient loading of Lake Tahoe from the Ward and Blackwood watersheds reflects a history of soil disturbance and vegetation removal. Logging, fire and stream channel diversion have been the dominant perturbations. Conservative extrapolation of annual loading data from this study to the entire basin indicates that algal nutrient levels in the lake probably have increased sufficiently in the century of mans intensive disturbance of the basin watersheds to account for increased phy- toplankton and periphyton production that have been measured and observed since 1958.

Patterns of nitrogen transport in streams of the Lake Tahoe Basin, California‐Nevada

In an effort to characterize the spatial and temporal patterns of nitrogen concentration and load in streams of the Lake Tahoe basin, we analyzed 10 years of data from 10 streams, developing discharge-concentration relationships and total load estimates for nitrate-N, ammonium-N, and organic N. The results indicate that (1) most of the nitrate transport occurs early in the snowmelt season or during large winter rainstorms; (2) dissolved organic nitrogen concentrations peak early in the runoff season, decline during snowmelt, and in some streams peak again during the summer low-flow period; (3) organic nitrogen accounts for over 90% of the total nitrogen load in basin streams; (4) averaged over 10 years and the area of 10 watersheds, the nitrogen flux rates in kg ha 21 yr 21 are 0.081 for nitrate-N, 0.017 for ammonium-N, 0.58 for dissolved organic N, and 0.47 for particulate organic N; (5) the variation in annual runoff explains most of the interannual and interwatershed variability in total nitrogen load; and (6) the dominance of organic nitrogen relative to nitrate-nitrogen in Tahoe basin streams contrasts with sites in eastern North America.

Hydrologic analysis for coastal wetland restoration

Increasing recognition of the value of tidal wetlands has led to interest in how to restore and enhance areas that have been modified by human activity. The policy of recognizing restoration or enhancement as mitigation for destruction of other wetlands is controversial. Once policy questions are separated from technical questions, the steps in a successful project are straightforward A key element in the design of a successful project is quantitative hydraulic and hydrologic analysis of alternatives. Restoration projects at two sites in California used a combination of empirical geomorphic relationships, numerical modeling, and verification with field observations. Experience with these and other wetland restoration projects indicates the importance of longterm postproject monitoring, inspection, and maintenance

Channel change, sediment transport, and fish habitat in a coastal stream: Effects of an extreme event

A study on sediment transport and channel change was conducted on Zayante Creek and the lower San Lorenzo River in Santa Cruz County, California. A rainstorm with a recurrence interval locally in excess of 150 years occurred during the study year, 1982 WY. Stream surveys indicated that significant aggradation occurred during and after the peak flood. Upper study reaches were substantially recovered after high flows of early April, but the lower study reaches still had significant filling of pools and burial of riffles by sand. Increases in width-depth ratio were minor and localized in upper reaches, but were significant in lower reaches. Large inputs of sand, primarily from landsliding, altered the sediment transport regime. A higher proportion of the bedload is now transported by lower flows than before the January event. Roads and sand quarries contributed significantly to sediment input to the stream. A proposed dam may alter the sediment transport regime of Zayante Creek. Mitigating the effects of this dam on downstream fish habitat may require occasional bankfull discharges.

Cumulative silvicultural impacts on watersheds: a hydrologic and regulatory dilemma

Because of the nature of watersheds, the hydrologic and erosional impacts of logging and related road-building activities may move offsite, affecting areas downslope and downstream from the operation. The degree to which this occurs depends on the interaction of many variables, including soils, bedrock geology, vegetation, the timing and size of storm events, logging technology, and operator performance. In parts of northwestern California, these variables combine to produce significant water quality degradation, with resulting damage to anadromous fish habitat.Examination of recent aerial photographs, combined with a review of public records, shows that many timber harvest operations were concentrated in a single 83 km2 watershed in the lower Klamath River Basin within the past decade. The resulting soil disturbance in this case seems likely to result in cumulative off-site water quality degradation in the lower portion of the Basin.In California, both state and federal laws require consideration of possible cumulative effects of multiple timber harvest operations. In spite of recent reforms that have given the state a larger role in regulating forest practices on private land, each timber harvest plan is still evaluated in isolation from other plans in the same watershed. A process of collaborative state-private watershed planning with increased input of geologic information offers the best long-term approach to the problem of assessing cumulative effects of multiple timber harvest operations. Such a reform could ultimately emerge from the ongoing water quality planning process under Section 208 of the amended Federal Water Pollution Control Act.

Projected 21st century trends in hydroclimatology of the Tahoe basin

With down-scaled output from two General Circulation Models (the Geophysical Fluid Dynamics Laboratory, or GFDL, and the Parallel Climate Model, or PCM) and two emissions scenarios (A2 and B1), we project future trends in temperature and precipitation for the Tahoe basin. With the GFDL, we also project drought conditions and (through the use of a distributed hydrologic model) flood frequency. The steepest trend (GFDL with A2) indicates a 4–5°C warming by the end of the 21st century. Trends in annual precipitation are more modest with a dip in the latter half of the 21st century indicated by the GFDL/A2 case, but not the others. Comparisons with the Palmer Drought Severity Index show that drought will increase, in part due to the declining role of the snowpack as a reservoir for soil moisture replenishment. Analysis of flood frequency for the largest watershed in the basin indicates that the magnitude of the 100-yr flood could increase up to 2.5-fold for the middle third of the century, but decline thereafter as the climate warms and dries. These trends have major implications for the management of land and water resources in the Tahoe basin, as well as for design and maintenance of infrastructure.

Nitrate transport in subalpine streams, Lake Tahoe Basin, California-Nevada, U.S.A.

Abstract This paper reports efforts to develop a linear model to relate nitrate-N concentration to two discharge variables and to fit the model to the data using multiple regression. The data set comprises >3100 mean daily discharge and nitrate-N concentration values representing 45 watershed-years. The goal was to compare the relative contribution to nitrate-N concentration of two dominant water types: “short flow-path water” that occurs during storms and snowmelt, and “long fiow-path water” or base flow. The first variable is a reciprocal function of discharge, derived from a mixing model for two water types in an open system. The second variable uses either cumulative water discharge or cumulative nitrate-N load for the water year. Stepwise linear regression was used to fit model parameters to the data. Both independent variables made a highly significant contribution to explaining the concentration variance. Values of R2 ranged from 0.22 to 0.45. For one catchment, the model was fitted to data for eight separate water years; it explained up to 80% of the variance in nitrate-N concentration. The results of this study indicate that the model can be used to distinguish anthropogenic nitrate sources from the ion pulse associated with early snowmelt, and to develop predictive models for estimating total load.

Modeling the transport of nutrients and sediment loads into Lake Tahoe under projected climatic changes

The outputs from two General Circulation Models (GCMs) with two emissions scenarios were downscaled and bias-corrected to develop regional climate change projections for the Tahoe Basin. For one model—the Geophysical Fluid Dynamics Laboratory or GFDL model—the daily model results were used to drive a distributed hydrologic model. The watershed model used an energy balance approach for computing evapotranspiration and snowpack dynamics so that the processes remain a function of the climate change projections. For this study, all other aspects of the model (i.e. land use distribution, routing configuration, and parameterization) were held constant to isolate impacts of climate change projections. The results indicate that (1) precipitation falling as rain rather than snow will increase, starting at the current mean snowline, and moving towards higher elevations over time; (2) annual accumulated snowpack will be reduced; (3) snowpack accumulation will start later; and (4) snowmelt will start earlier in the year. Certain changes were masked (or counter-balanced) when summarized as basin-wide averages; however, spatial evaluation added notable resolution. While rainfall runoff increased at higher elevations, a drop in total precipitation volume decreased runoff and fine sediment load from the lower elevation meadow areas and also decreased baseflow and nitrogen loads basin-wide. This finding also highlights the important role that the meadow areas could play as high-flow buffers under climatic change. Because the watershed model accounts for elevation change and variable meteorological patterns, it provided a robust platform for evaluating the impacts of projected climate change on hydrology and water quality.

Management plan for an alkali sink and its endangered plantCordylanthus palmatus

Cordylanthus palmatus is a hemiparasitic annual of the family Scrophulareacae. It is on both the federal and state lists of endangered species. Only four widely separated populations remain, all of them in alkali sinks, where the plant thrives in saline-sodic soils. The largest population is at Springtown, Alameda County, California. This article reports on efforts to develop a management plan for both the plant and the alkali sink ecosystem. The plan is based on: (1) characterization of hydrology, soils and geomorphology of the site; (2) characterization of the land use impacts to the site; (3) analysis of plant distribution in relation to gradients of elevation and soil chemistry; (4) studies on water potential and water stress inCordylanthus palmatus and associated species. On the basis of this plan, both the State of California and private groups are cooperating to create, restore, and manage a preserve in the Springtown Alkali Sink.

The Road to Erosion

An increased awareness of the devastation of soil erosion has provided the impetus behind judicial and legislative reforms. The situation of watershed degradation in the North Coastal region of California is followed. The region is drained by a series of rivers that generally flow northwest, following zones of structural weakness and faulting. Vegetation is a complex mosaic of grassland, brush, hardwoods, and conifers. The combination of intense precipitation, high peak flows, human impacts, and structurally unstable rock has given the river basins of the North Coast some of the highest erosion rates in the world. The calculated average erosion rates based on sediment yield from the North Coast rivers vary somewhat, depending on the period of record and assumptions used. These erosion rates are 10 to 100 times the rates of other river basins in the US. They are rivaled only by the phenomenal erosion rates in large river basins of Asia.