Department of
Environmental Quality

Hydrogeology is the study of water under the earth’s surface. The hydrogeology of the Rathdrum Prairie is influenced mainly by the many ancient glacial floods in the area that deposited large quantities of sediment that form the Rathdrum Prairie Aquifer (RPA) today. The aquifer is mainly made up of coarse sand, gravel, cobbles, and boulders, although areas of fine-grained silt and clay sediments can also be found.

Size of the Aquifer

The map below shows how far the RPA extends laterally (north, south, east, and west), according to the U.S. Geological Survey (2005). Note that the map shows the larger Rathdrum-Spokane Aquifer, which extends into Washington. The lateral limits of the RPA are typically where the unconsolidated sediments are next to a lake or contact the bedrock, or where a ground water divide is present. A ground water divide is an area underground in which the ground water flows in opposite directions, such as in the Spirit or Hoodoo Valleys.

Well driller's reports and gravity surveys indicate how deep the RPA is. However, it is difficult to find wells over the RPA that have been drilled deep enough to hit bedrock (which would mean the bottom of the aquifer has been reached). Wells this deep are rare because the aquifer is permeable, allowing most wells to pump enough water at a shallow depth.

To determine the thickness of the RPA, gravity measurements were used. The gravity field at the earth's surface has extremely small variations, depending on the material below. The gravity field can be measured with an instrument called a gravimeter. The earth's gravity field is equal to one g; a gravimeter is capable of measuring differences in the earth's gravity field of one one-hundred-thousandth (0.00001) of a g.

Existing gravity data for the Rathdrum Prairie were compiled, and gravity was measured at an additional 472 locations. The gravity data were then analyzed and processed to convert the gravity measurements into depth to bedrock. The depths to bedrock were plotted like topographic lines on a map and can be seen in the figure below. The depth to bedrock in the RPA varies between 0 and 420 meters (1,378 feet).

Composition of the Aquifer

The RPA is made up of different quantities of silt, sand, and gravel. Some areas of the Rathdrum Prairie have more gravel, while others have more silt or clay. The different compositions affect the ability of the aquifer material to transmit water. The aquifer material’s ability to transmit water is called hydraulic conductivity and can be measured in units of length per time, such as feet per day (ft/d). Hydraulic conductivity is not a measure of how fast water moves in the aquifer, however. Typically, sand and gravel will have a hydraulic conductivity between 50 and 5,000 ft/d. As the figure above shows, the hydraulic conductivity of material in the RPA can be very high—over 22,000 ft/d. This is due to the ancient massive floods that largely washed away fine material and left behind large, coarse material like sand and gravel, which easily transmit water.

Aquifer Recharge

Water enters the RPA from various sources, including precipitation, runoff from the surrounding bedrock areas, seepage from lakes and rivers, and return flows from irrigation (irrigation water not used by plants that works its way back to the aquifer). Precipitation on the Rathdrum Prairie and surrounding areas varies by location, between 16 and 67 inches per year. Average precipitation for the Coeur d'Alene area is about 25 inches per year (see map below). When precipitation lands on the ground, some evaporates, some is used by plants, and the rest seeps into the aquifer or forms runoff from the surrounding upland areas. Only 20-25% of precipitation is actually available for recharge to the aquifer, since the rest either evaporates or is used by plants.

Some of the water that enters the RPA comes from surrounding upland areas called tributary areas. The precipitation that falls in these areas flows down to the aquifer as surface flow in streams and underground as ground water. The water recharges the aquifer by seeping into the ground around the edges of the aquifer.

In the RPA, ground water flows from the north, near Spirit and Hoodoo Valleys and Lake Pend Oreille, southwards toward Post Falls. The aquifer is recharged along the way with runoff from the bedrock upland areas and the lateral lakes such as Hayden, Twin, and Spirit Lakes. Additional water flows westward from Coeur d'Alene Lake, eventually flowing toward the Idaho-Washington state line.

Water enters and leaves the aquifer in a number of different ways and in varying amounts. The figure below shows the total amounts flowing into and out of the aquifer in both Washington and Idaho. Two of the largest contributors to recharge of the aquifer in Idaho are seepage from the Spokane River and lakes along the aquifer boundary. Surprisingly, the amount of seepage from the two largest lakes in North Idaho, Lake Coeur d'Alene and Lake Pend Oreille, is relatively small compared to the amount of seepage from smaller lakes such as Hayden and Spirit Lakes. The largest outflows from the aquifer in both Idaho and Washington are seepage to the Spokane River and ground water pumping for drinking water and irrigation.

In Idaho, the Spokane River is largely losing water to the aquifer. In Washington, the river both loses and gains water from the aquifer. (See figure below.)

The reason the Spokane River both loses and gains water from the RPA in Washington is mainly due to the elevation change from Idaho to Washington. In Coeur d’Alene, the ground water is about 100 to 200 feet below the ground surface, but in Spokane, it varies from about 20 to 30 feet below ground surface to 0 feet at the Spokane River. (See figure below.) The depth to ground water is much less in Washington because as you move west from Idaho to Washington, the ground surface elevation decreases much faster than the slope of the water table.