INTRODUCTION

Since the early 1970's, monitoring stream water quality in the Lake Tahoe Basin has been done by the Tahoe Regional Planning Agency (TRPA), the U.S. Geological Survey (USGS), the Tahoe Research Group (TRG) of the University of California at Davis, and State and local agencies. The Lake Tahoe Interagency Monitoring Program (LTIMP) was began in 1978 and described in Rowe (2000), Boughton and others (1997), and Rowe and Stone (1997). A tributary-monitoring program, a cooperative effort by TRPA, USGS, and TRG, began in water year 1988 and continues currently (2002). Fourteen of the 63 watersheds in the Lake Tahoe Basin (fig. 1) are monitored in the LTIMP network including a total of 41 tributary stations (table 1).

Data collected from the LTIMP network stations include streamflow, water-quality parameters, nutrients, and sediment (table 2). Schedules for sampling were determined by the type of station (primary, secondary, or miscellaneous). A primary station has a continuous streamflow gage and 25-40 water-quality samples are collected annually at near mouth locations in 10 watersheds. A secondary station has a continuous streamflow gage and 20-30 water-quality samples are collected annually at 10 multiple-station sites in 5 of 10 watersheds. Samples are collected from primary and secondary stations during monthly baseline periods and runoff events. A miscellaneous station is a sampling station, urban-runoff sampling station, or water-temperature network station. The 12 miscellaneous sampling stations are ungaged and 6-10 water-quality samples are collected annually (mainly during snowmelt and storm-runoff periods). Sampling at the three station types was emphasized during spring snowmelt, rain-on-snow events, summer thunderstorms, and fall rain events, whether they occurred during day or night.

Background

Lake Tahoe is within the crest of the Sierra Nevada Range and lies within the states of California and Nevada (fig. 1). Lake Tahoe is described as an outstanding natural water resource and is famous for its alpine setting and deep, clear waters. Protection of these clear waters has become very important in the past half century, because water clarity has been decreasing by about 1 ft/yr (Goldman and Byron, 1986). This decrease in clarity is mainly due to human activities, which have increased dramatically in the Lake Tahoe Basin since 1960.

Increased nutrient concentrations within Lake Tahoe are considered the primary cause of algal growth, and thereby loss of lake water clarity. Suspended sediment also is of concern, because nutrients attach to and are transported by sediment particles. Within the Lake Tahoe Basin, one of the major pathways for nutrient and sediment transport to the lake is streamflow. Transport of those materials has been accelerated progressively by development activities such as urbanization of wetlands and steeply sloping terrain around the lake margin.

Public concern for the clarity of Lake Tahoe has increased over the years. In 1986 and 1996, voters in Nevada passed bond acts to fund construction projects to reduce erosion and the transport of nutrients and sediments to Lake Tahoe in the Nevada portion of the Lake Tahoe Basin. The Lake Tahoe Interagency Monitoring Program (LTIMP) was developed to collect water-quality data in the basin. LTIMP water-quality data and streamflow data are available to the public through several USGS internet web sites (app. 1).

Acknowledgments

Dr. John Reuter, Patty Arneson, and Loren Hatch, all of the University of California at Davis, contributed long-term nutrient data.

Purpose and Scope

This report presents summaries of the streamflow, water-quality, and nutrient and suspended-sediment concentrations data for 41 selected tributary stations in 14 watersheds. Estimated monthly loads, yields, and trends for nutrients and suspended sediment also are presented for 20 sampling stations in 10 watersheds in the Lake Tahoe Basin. These estimates were derived using concentration and daily streamflow data combined from the LTIMP study (water years 1988-98; from October 1, 1987, to September 30, 1998) and from previous monitoring data. All annual data used in this report are based on water years. Estimations of load are reported here only when daily streamflow records exist. Comparisons between the 10 watersheds and within watershed comparisons in 5 multiple-site watersheds also are presented.