Introduction

The shoreline of Coeur d’Alene Lake in northern Idaho has been developed for residential and commercial land uses. In addition, communities bordering the lake have experienced rapid population growth. Water managers and private landowners are concerned about the effects of rapid growth and increasing water demands from the lake on the available surface-water supplies. Of particular concern is the possibility of declining lake levels because of new residential and commercial developments. An updated surface-water budget for Coeur d’Alene Lake is needed to help resource managers determine how growth and rising water demands will affect lake water levels. Most previous studies of Coeur d’Alene Lake focused on water-quality issues in relation to nutrient loads and trace element enrichment from contributing drainage basins, primarily the mining districts in the Coeur d’Alene River basin (Wood and Beckwith, 2008). The last water budget for Coeur d’Alene Lake was calculated for water years 1991 and 1992 (Woods and Beckwith, 1997).

In 2007, the U.S. Geological Survey (USGS) began a cooperative project with the Idaho Department of Water Resources (IDWR) to develop annual surface-water budgets and a mean annual surface-water budget for Coeur d’Alene Lake for water years 2000 through 2005. A water year is defined as a year beginning October 1 and ending September 30 of the following year—thus, water year 2000 begins on October 1, 1999, and ends on September 30, 2000. A water budget helps resource managers to quantify the components of a surface-water budget for a lake and to identify any adverse effects that rapid growth may have on the water levels of a lake. A decline or rise in the water level of the lake is indicated by a respective negative or positive net change in the lake storage term of the water budget.

Purpose and Scope

This report presents the annual water budgets and summarizes the components used to develop water budgets for Coeur d’Alene Lake for water years 2000–2005. The components of the water budget considered in this study are surface-water inflow and outflow, precipitation, evaporation, change in lake storage, estimate of consumptive use (for withdrawals from the lake for domestic and municipal uses and from wells close to the lakeshore), streamflow loss to the Spokane Valley-Rathdrum Prairie aquifer from the Spokane River between the outlet of Coeur d’Alene Lake and Post Falls Dam, seepage loss from Coeur d’Alene Lake to the aquifer, a residual value that includes error in estimates and measured data, and unmeasured components of the water budget. For example, calculations of groundwater fluxes between the lake and groundwater systems around the lake, or evapotranspiration from vegetation around the lake, which are both part of a water budget were not quantified in this study and are included in the residual value.

Description of Study Area

Coeur d’Alene Lake is in northern Idaho within the 6,680 mi² Spokane River basin (fig. 1). The lake is the second largest in the State, with a drainage area of about 3,740 mi² (2.4 million acres). The lake is bordered by forested mountains to the east and a combination of forest and cultivated land to the west. Most population, recreational use of the lake, and lakeshore development are in Kootenai County (Woods and Beckwith, 1997). Most surface-water inflows to the lake are from two large rivers, the Coeur d’Alene and the St. Joe, which enter the lake from the east and southeast, respectively (fig. 1). Smaller tributaries to the lake include Wolf Lodge Creek, Carlin Creek, Plummer Creek, and Fighting Creek. The city of Coeur d’Alene borders the northern shore of the lake and the smaller communities of St. Maries, Harrison, and Plummer are to the south, east, and west, respectively. The lake is almost completely surrounded by campgrounds, marinas, and hundreds of single-family homes, many of which are summer homes occupied seasonally.

At a full summer pool elevation of 2,132 ft (NAVD 88), Coeur d’Alene Lake covers about 50 mi² and has 135 mi of shoreline and a north-south length of about 22 mi. The outlet of the lake is at the northern end near the city of Coeur d’Alene, where water flows into the Spokane River. Lake levels are regulated by Post Falls Dam, about 11 mi downstream on the Spokane River near Post Falls, Idaho. The southern end of the lake is under the jurisdiction of the Coeur d’Alene Tribe. The southernmost areas of the lake are shallow because prior to the construction of Post Falls Dam in 1906 the area was primarily emergent wetlands.

Most population in the study area is concentrated in the four communities of Coeur d’Alene, Harrison, St. Maries, and Plummer. In 2006, the estimated total population of these communities was 45,000 (U.S. Census Bureau, 2008; fig. 2). The city of Coeur d’Alene is the largest of the four communities, with approximately 41,000 residents in an area of about 14 mi². The combined total population of St. Maries, Plummer, and Harrison was 3,980 people in 2006, less than 10 percent of the population of Coeur d’Alene. From 1980 to 2006, the total population in communities surrounding the lake increased 88 percent.

The climate of the Coeur d’Alene Lake drainage area is mild and arid during summer, and is cold and wet during winter. Approximately 70 percent of the total annual precipitation falls between October and April, and about one-half of that precipitation falls as snow. The driest months are July and August. Mean annual precipitation ranges from 20 in. near the lake to 50–60 in. in the higher elevations of the drainage basin (Molnau, 2000). Rain-on-snow events are not uncommon during winter; heavy runoff occurs in April and May, but can continue into June. Precipitation-bearing storms usually originate from the northwest over the Pacific Ocean, but occasionally storms will originate from the north over Canada. Mean monthly temperatures (based on long-term records at all three weather stations) range from -1.6 to 20°C the coolest monthly averages occur in January and the warmest averages occur in July. Mean summer temperatures are about 16°C, and mean winter temperatures are about -1.1°C (Western Regional Climate Center, 2008).

After a near-normal start in water year 2000, northern Idaho experienced a cycle of wet and dry years between 2001 and 2005. The following percentages are for average precipitation and streamflow conditions as compared to 30-year averages for major drainage basins in northern Idaho (Panhandle). Water year 2000 was a near-normal year with 102 percent of average precipitation and 104 percent of average streamflow. However, water year 2001 was the driest year during the study period, with 59 percent of average precipitation and only 42 percent of average streamflow. The cycle turned wet in water year 2002 and was the wettest year in the study period with 110 percent of average precipitation and 125 percent of average streamflow. Drier conditions returned during water year 2003 with 84 percent of average precipitation and 75 percent of average streamflow. Water year 2004 had mixed conditions with above-average precipitation (104 percent), but below-average streamflow (74 percent). Water year 2005 had below-average precipitation (82 percent) and streamflow (76 percent).