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CHANGES IN WATER-QUALITY CONDITIONS IN LEXINGTON RESERVOIR, SANTA CLARA COUNTY, CALIFORNIA, FOLLOWING A LARGE FIRE IN 1985 AND FLOOD IN 1986 By Marcus J. Taylor, Johnevan M. Shay, and Scott N. Hamlin U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 92-4172 Prepared in cooperation with the SANTA CLARA VALLEY WATER DISTRICT CO CO I CO t I o CO Sacramento, California 1993 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY DALLAS L. PECK, Director Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. For sale by the Books and Open-File Reports Section U.S. Geological Survey Federal Center, Box 25425 Denver, CO 80225 For additional information write to: District Chief U.S. Geological Survey Federal Building, Room W-2233 2800 Cottage Way Sacramento, CA 95825 CONTENTS Abstract 1 Introduction 1 Study design 3 Description of study area 4 Reservoir and stream 4 Climate 4 Fire and flood 6 Changes in water-quality conditions following the fire and flood 6 Physical and chemical characteristics 6 Water-profile data 6 Spring 6 Summer 8 Autumn 9 Major ions 9 Trace elements 11 Plant nutrients 13 Biological characteristics 15 Phytoplankton composition and chlorophyll-^ concentrations 15 Similarity index 18 Trophic State Index 19 Summary 21 References cited 22 FIGURES 1. Map showing location of study area 2 2. Hydrograph showing reservoir volume, sampling dates, and percentage of maximum capacity at time of sampling for Lexington Reservoir 3 3. Boxplot showing rainfall at Alamitos in February 1978-87 5 4. Profiles showing water transparency, temperature, dissolved oxygen, pH, and specific conductance at Lexington Reservoir for March 1980, 1984, and 1986; April 1985 and 1987; and May 1980 and 1986 7 5. Profiles showing water transparency, temperature, dissolved oxygen, pH, and specific conductance at Lexington Reservoir for June 1985 and 1987, July 1987, June 1978 and 1986, August 1978 and 1986, September 1980, and October 1986 10 6. Trilinear diagram showing major-ion composition of water in Lexington Reservoir and Los Gatos Creek upstream from Lexington Reservoir for spring, summer, and autumn months for water years 1978-80 and 1984-86 13 7. Graph showing similarity indexes for phytoplankton composition in Lexington Reservoir before and after the fire and flood, 1978-87 19 Contents TABLES 1. Sampling dates selected to represent seasonal conditions in Lexicon Reservoir before and after the fire and flood 4 2. Rainfall recorded at Alamitos 5 3. Significant changes in mean concentrations of major ictisi and trdce elements in Los Gates Creek and Lexington Reservoir, before and after the fire and flood 14 4. Significant changes in mean concentrations of nutrients at Lexington Reservoir for paired samples before and after the fire and flood 15 5. Significant changes in mean concentrations of plant nutrients in Lexington Reservoir and Los Gatos Creek averaged before and after the fire and flood 16 6. Co-dominant genera and changes in phytoplankton numbers and chlorophyll-a concentration in Lexington Reservoir, before and after the fire and flood 17 7. Tropic State Indexes for Lexington Reservoir 20 Conversion Factors and Vertical Datum Within this report, both metric (International System or SI) and inch-pound units are used. Depth and water-quality characteristics are reported in metric units, and physical characteristics , velocity, discharge, area, and altitude are reported in inch-pound units. Multiply By To obtain acre 0.4047 hectare acre-foot (acre-ft) 1,233 cubic meter foot (ft) 0.3048 meter inch (in.) 25.4 millimeter meter (m) 3.281 foot mile (mi) 1.609 kilometer square mile (mi2) 2.590 square kilometer ton per acre-foot 824 milligram per liter Temperature is given in degrees Celsius (°C), which can be converted to degrees Fahrenheit (°F) by the following equation: °F=1.8(°C)+32. Vertical Datum Sea Level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929~a geodetic datum derived from a general adjustment of the first-order level nets o the United States and Canada, formerly called Sea Level Datum of 1929. IV Conversion Factors and Vertical Datum CHANGES IN WATER-QUALITY CONDITIONS IN LEXINGTON RESERVOIR, SANTA CLARA COUNTY, CALIFORNIA, FOLLOWING A LARGE FIRE IN 1985 AND FLOOD IN 1986 By Marcus J. Taylor, Johnevan M. Shay, and Scott N. Hamlin Abstract opposite trends in phytoplankton numbers and chlorophyll-a concentrations may be attributed In July 1985, a wildfire burned 22 square partly to the difference in cell volumes associated miles of steep, predominantly chaparral-covered with individual genera. terrain in the 37 square miles of the drainage basin upstream from Lexington Reservoir, Cali­ fornia. A series of subtropical storms produced INTRODUCTION 25.5 inches of rain at the reservoir during Lexington Reservoir in Santa Clara County, Cali­ February 12-19, 1986, with maximum intensities fornia (fig. 1), was formed by the construction of for 1 hour exceeding 0.8 inch. Resulting runoff Lexington Dam across Los Gatos Creek and is used filled the empty Lexington Reservoir within 36 primarily for water conservation. Surface-water run­ hours. The changes in water-quality conditions off is impounded in the reservoir during the rainy of Lexington Reservoir and its outlet stream Los season and released to downstream recharge areas Gatos Creek that occurred as a result of the com­ during the dry summer months. Water released from bined fire and flood are characterized through Lexington Reservoir supplements the natural recharge analysis of data collected before and after the fire of the ground-water basin, helping to satisfy increased and flood. ground-water pumping demands. Along with water conservation, the reservoir's beneficial uses include After the flood in the spring of 1986, water of recreation and sport fishing. low conductivity, low chloride, and high turbidity entering the reservoir slowed the development of In 1978, the Santa Clara Valley Water District phytoplankton and suppressed population num­ began a new water-quality monitoring network revised bers by two orders of magnitude. This biolog­ from an earlier program because of changing water- ical retardation occurred despite increases in the and land-use activities. The increasing complexities concentrations of nitrogen and phosphorus of one of regulations governing the quality of water distrib­ and two orders of magnitude, respectively. Dur­ uted for various purposes made the existing network ing the growing season, water transparency in­ outmoded (Pederson and others, 1978). The revised creased as did the phytoplankton population, reservoir-monitoring network resulted in the priority which rose to three orders of magnitude greater ranking of drainage basins with reservoirs and sam­ than that recorded during summer and autumn pling of reservoirs and their major tributaries on a months previous to the fire and flood. Corre­ 3-year rotational basis. Lexington Reservoir was the spondingly, similarity indexes indicate that first reservoir sampled under the new water-quality phytoplankton succession recovered by summer monitoring network designed by the Geological and followed a pattern similar in composition to Survey and the Santa Clara Valley Water District. that of previous years. Trophic State Indexes From 1978 through 1987, the reservoir was sam­ calculated using values of water transparency, pled on 22 dates, with no data available from 1981 to chlorophyll-a, and total phosphorus decreased by 1983. Physical and chemical data collected include 2 to 16 points, or about a 10- to 75-percent specific conductance, pH, water temperature, water decrease in biomass production for 1986. The transparency, dissolved oxygen, major chemical ions, Introduction 1 San Francisco FS San Jose MAP AREA 122°00' 37°15' - 0 50 100 MILES Vasona 50100 KILOMETERS Reservoir Monte Sereno EXPLANATION AREA OF BURN WATER-SAMPLING STATION METEOROLOGICAL STATION Redwood Estates 37°07'30" Base from U.S. Geological Survey San Jose 1:100,000,1986 3 MILES 0 1 3 KILOMETERS Figure 1. Location of study area. 2 Water-Quality Conditions in Lexington Reservoir, California, Following a Large Fire in 1985 and Flood in 1986 nutrients, and selected trace constituents. Trophic two stations were sampled on Los Gatos Creek, one characteristics data, including phytoplankton numbers immediately upstream from the reservoir and one and chlorophyll-a concentrations, also were collected. downstream from the reservoir. The station down­ The program originally called for quarterly sampling stream from the reservoir was monitored only for 1 but was restructured in 1985 because the reservoir year after the storm (fig. 1). was frequently empty or was too low to permit launching of a boat for sampling. Starting in 1985, a The data analyzed in this report were collected by decision was made not to sample the reservoir unless the methods described by Greeson and others (1977), the volume was significantly different than it was U.S. Geological Survey (1977), Skougstad and others during the same season of the previous year. This (1979), and Britton and Greeson (1988), and were decision, in addition to occasional equipment failure published by water year by the U.S. Geological Sur­ and inability to launch, resulted in an incomplete set vey (1979,19810,19816,1982), Markham and others of monitoring data for the period of record. (1983), and Anderson and others (1984, 1985, 1987, 1988a, 19886). Iwatsubo and others (1988) published In July 1985, a wildfire burned 22 mi2 or about an interpretive report describing data collected from 80 percent of the steep, predominantly chaparral- 1978 through 1980. covered terrain in the 37 mi2 of the drainage basin upstream from Lexington Reservoir. During Febru­ Temperature, pH, specific conductance, and dis­ ary 12-19, 1986, 25.5 in. of rain fell at the reservoir solved oxygen were measured using a multiparameter with maximum intensities for 1-hour intervals exceed­ probe at intervals of 1 m.

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