USGS Home
Contact USGS
Search USGS

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5168

Back to Table of Contents

Introduction

From the late 1880s to the early 1980s, the mining district in the South Fork Coeur d’Alene River valley was among the Nation’s largest producers of silver, lead, zinc, and other metals. The mining and ore-processing methods used in the “Silver Valley” to extract this wealth produced large quantities of waste material containing environmentally hazardous contaminants such as cadmium, lead, and zinc. Much of this material was discharged directly to or washed into the South Fork Coeur d’Alene River and its tributaries. The beds, banks, and floodplains of the Coeur d’Alene River, Coeur d’Alene Lake, and to a lesser extent the Spokane River, contain large quantities of metal-contaminated sediments that are transported downstream and dispersed by hydrologic processes, including through Coeur d’Alene Lake, into the Spokane River, and into the State of Washington. Large quantities of metal-contaminated sediment presently reside in the bed of Coeur d’Alene Lake that, unless removed, will require long-term management actions to maintain them in place without risk to human health and the environment. The extent and magnitude of mining associated environmental contamination throughout the South Fork and main stem Coeur d’Alene River, Coeur d’Alene Lake, and Spokane River basins has resulted in environmental cleanup (“Superfund”) and Natural Resources Damage Assessment and Restoration (NRDAR) litigation actions under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) that are among the largest, most complex, longest in duration, and costliest in the Nation. The potential for nutrient enrichment and subsequent eutrophication of Coeur d’Alene Lake also are of considerable concern because of land-use activities throughout the drainage basin and along the shoreline, as well as intensive recreational use in the lake (Woods and Beckwith, 1997). A nutrient load study conducted in 1975 as part of the National Eutrophication Survey led to the determination that Coeur d’Alene Lake was mesotrophic, or moderately productive (U.S. Environmental Protection Agency, 1977), which prompted continued observation of the lake’s trophic status.

Comprehensive studies of Coeur d’Alene Lake and the surrounding watershed were conducted in the late 1980s and early 1990s by the U.S. Geological Survey (USGS) and Coeur d’Alene Tribe in cooperation with Idaho Department of Environmental Quality (IDEQ) (Woods, 1989; Horowitz and others, 1993, 1995a, 1995b; Woods and Berenbrock, 1994; Woods and Beckwith, 1997).

In 1998, the U.S. Environmental Protection Agency (USEPA) began a Remedial Investigation/Feasibility Study (RI/FS) to assess mining associated contamination outside the original 21 mi² Bunker Hill Mining and Metallurgical Complex Superfund Site surrounding the communities of Kellogg and Smelterville, Idaho. USGS researchers were involved in the RI/FS by establishing and operating the Coeur d’Alene Basin Environmental Monitoring Program to monitor hydrological conditions and constituent concentrations and transport throughout the Coeur d’Alene River basin, and by conducting additional, highly focused limnological sampling of contaminant transport through the lake and geochemical research of contaminant flux at the sediment-water interface. In 2000, the U.S. 9th Circuit Court of Appeals ruled that the Bunker Hill Site includes all areas in the Coeur d’Alene Lake/Spokane River basin (the Basin) where hazardous substances have come to be located. In 2002, the USEPA issued a Record of Decision (ROD) (U.S. Environmental Protection Agency, 2002) for Operable Unit 3 (OU3) of the Bunker Hill Superfund Site describing about 30 years of prioritized remedial actions for cleanup throughout the basin.

The selected remedy for OU3 did not include actions for Coeur d’Alene Lake. This decision was deferred pending the development and effective implementation of a revised Lake Management Plan to protect water quality and to effectively contain and prevent mobilization of metals from lakebed sediments. A draft Lake Management Plan was developed in June 2008 (IDEQ and Coeur d’Alene Tribe, 2008). When finalized, it will be implemented by State, tribal, Federal, and local governments outside of the formal CERCLA process using separate regulatory authorities and funding.

Purpose and Scope

This report presents insights into the limnological functioning of Coeur d’Alene Lake based on information developed from two large-scale limnological studies of the lake conducted by the USGS and the Coeur d’Alene Tribe during calendar years 1991–92 and water years 2004–06. The data collection and evaluation activities of the 2004–06 limnological studies discussed in this report were designed to provide information for the Bunker Hill Superfund Site OU3 and the Lake Management Plan (specified in the OU3 ROD to be developed and implemented instead of a formal CERCLA remedy).

The 2004–06 study included sampling and evaluation of limnological characteristics not covered by the 1991–92 study. An important goal of the 2004–06 study was to better understand the interaction of physical, chemical, and biological processes in the lake to aid in the interpretation of future water-quality trends in Coeur d’Alene Lake as remedial actions are undertaken under the OU3 ROD and the Lake Management Plan. The study design sought to evaluate the interaction of trace metals, nutrients, and lake productivity during water years 2004–06 using highly focused sampling of physical, chemical, and biological characteristics over a range of spatial and temporal conditions. The study’s technical scope covered the following data collection and evaluation activities: (1) mass balances of trace metals and nutrients, (2) fate and transport of nutrients and quantification of lake productivity, and (3) fate and transport of trace metals.

In this report, important limnological characteristics measured by both studies are compared to gain understanding of the complex interaction of physical, chemical, and biological processes in the lake and the resulting water-quality conditions, to assess temporal and spatial differences, and to identify potential changes and trends in lake water quality over time. The insights gained from the two studies can be used in the decision-making process for managing Coeur d’Alene Lake water quality and contaminants in the basin.

Description of Study Area

Coeur d’Alene Lake—Idaho’s second largest lake—is in northern Idaho (fig. 1) in the 17,300-km² Spokane River basin. The lake was created about 18,000–13,000 years ago, after a series of outburst floods from glacial Lake Missoula deposited about 100 m of coarse gravel deposits in the Rathdrum Prairie-Spokane River Valley, damming the Pleistocene Spokane River and its two primary tributaries, the Coeur d’Alene and St. Joe Rivers (URS Greiner, Inc., and CH2M Hill, Inc., 2001). Present-day Coeur d’Alene Lake covers 129 km² at full summer pool elevation of 649.8 m (NAVD 88) and is about 35-km long in a north-south orientation. The maximum width is about 3.7 km, and the maximum depth is about 64 m. The volume is 2.8 km³ at full pool (Woods and Berenbrock, 1994).

The southern end of the lake, under Tribal jurisdiction, is contiguous with four shallow lakes (Benewah, Chatcolet, Hidden, and Round Lakes) that were primarily emergent wetlands prior to the 1906 construction of Post Falls Dam on the Spokane River downstream of the lake outlet. Dam operation provides storage for hydroelectric power generation by maintaining the lake surface elevation approximately 2.1-m higher than normal, following spring runoff, through late summer as the lake recedes. By delaying lake recession until late summer or early autumn, dam operation has created large expanses of shallow open water areas, increased habitat for aquatic macrophyte growth, and altered the natural levees, lateral lakes, and wetlands ecosystems in the lower reaches of the Coeur d’Alene and St. Joe Rivers.

The drainage area of Coeur d’Alene Lake is about 9,690 km²; most (90 percent) of the lake’s inflow is delivered by the Coeur d’Alene and St. Joe Rivers (Woods and Beckwith, 1997). Coeur d’Alene Lake outflows to the Spokane River, a tributary of the Columbia River. The drainage area of the lake is comprised mostly of mountains covered in coniferous forests and deep, intermontane valleys. The Coeur d’Alene and St. Joe Rivers originate in the Coeur d’Alene and St. Joe Mountains, respectively, which are sub-ranges of the Bitterroot Mountains that form the Idaho-Montana border. Elevations range from about 650 m (NAVD 88) near the lake outlet to about 2,090 m (NAVD 88). About 78 percent of the land cover in the drainage area is forest (coniferous, sparse, or recovering harvest), and rangeland and agriculture account for about 7 and 5 percent, respectively. The remaining 10 percent of the land cover, in order of largest to smallest area, is clear cut/barren land, small water bodies, wetlands, and urban and mine areas.

Annual precipitation in the drainage basin is higher than other areas in Idaho. Snowfall between October and April comprises about 70 percent of the annual precipitation. The influence of Pacific Maritime conditions can produce large rain-on-snow events during winter. The mountainous topography of the basin affects the spatial distribution of precipitation. For example, mean annual precipitation is about 64 cm at the lake, whereas mean annual precipitation is about 97 cm at Wallace, Idaho, a town in the drainage basin, which is about 75 km east and 250-m higher in elevation than the lake. Ambient temperature varies similarly, and although winter temperatures at Coeur d’Alene Lake often are below freezing, the lake does not typically freeze except at the shallow southern end.

The Coeur d’Alene River (drainage area 3,810 km²) discharges into the southern one-third of the lake near Harrison, Idaho, and is comprised of three major reaches: the North Fork, the South Fork, and below the confluence of the North and South Forks (not shown in figures). Primary land-use activities in the North Fork drainage basin include recreation, forestry, and agriculture, with limited historical placer and hard-rock mining. Extensive hard-rock mining, ore processing, and transportation activities occurred in the South Fork Coeur d’Alene River drainage basin. Known as the “Silver Valley”, the South Fork basin was among the Nation’s largest producers of silver, lead, and zinc for almost a century; currently, it includes one of the Nation’s largest Superfund sites with only limited mining and ore processing. The St. Joe River (drainage area 4,520 km²) discharges into the extreme southern end of the lake, several miles downstream of the confluence of the St. Joe and St. Maries Rivers near St. Maries, Idaho. Recreation, forestry, and agriculture are the dominant land uses in the St. Joe River drainage area, and little mining activity has occurred. Urban land use is relatively low (less than 1 percent as of 1997) in the St. Joe and St. Maries River basins (Woods and Beckwith, 1997). However, considerable population growth and land development is occurring near the lake, including several proposed golf resort communities and marinas.

Back to Table of Contents