The Spokane Valley-Rathdrum Prairie Aquifer
The sole source of water for most of the people in Spokane County, Washington and Kootenai County, Idaho, is a high quality underground water body called the Spokane Valley-Rathdrum Prairie Aquifer, and it is also commonly known as the Rathdrum-Spokane Aquifer. Discovered in 1895, this Aquifer has become one of the most important resources in the region, supplying drinking water to more than 500,000 people. The Aquifer has been studied in considerable detail since 1977, and the results of these investigations have produced programs and regulations designed to ensure this aquifer will remain a valued and protected resource for future generations.
- The aquifer has one of the fastest flow rates in the United States, flowing as much as 50 feet per day in some areas. In comparison, a typical aquifer has a flow rate between one-quarter inch and five feet per day.
- The Aquifer sediments range from about 150 feet to 600 feet deep.
- The Aquifer covers 322 square miles in two states.
- The average daily water withdrawal is about 146 million gallons.
- The volume of the entire aquifer is about 10 trillion gallons, making it one of the most productive aquifers in the United States.
The Spokane Valley-Rathdrum Prairie Aquifer flows beneath a broad valley that slopes downward from Lake Pend Oreille to downtown Spokane, losing almost 700 feet in elevation. The basalt formation that creates Spokane Falls diverts the Aquifer flow north after downtown Spokane. Five Mile Prairie splits the Aquifer flow from downtown Spokane with the Hillyard Trough to the east and the Spokane River valley to the west. North of downtown Spokane the surface elevation rises and then drops steeply at the confluence of the Spokane and Little Spokane Rivers. In general, the higher the surface elevation, the greater the depth to the Aquifer.
The Spokane-Coeur d’ Alene region has warm, dry summers and cool, moist winters. The area’s climate ranges from sub-humid in the mountains to semiarid in the Spokane Valley and Rathdrum Prairie. July is the warmest month (85.8 degrees Fahrenheit average maximum in Spokane) and January the coolest month (34.3 degrees Fahrenheit average minimum in Coeur d’ Alene). Temperatures in Coeur d’ Alene are usually a few degrees cooler than in Spokane. Average annual precipitation varies considerably over the region: Spokane averages 17.62 inches with Coeur d’ Alene averaging 25.45 inches, while east of Coeur d’ Alene in the Bitterroot Mountains, part of the Aquifer recharge area, annual precipitation is more than 70 inches. About sixty percent (60%) of the annual precipitation occurs during the five-month period November through March, and much of it falls as snow, especially in the mountains.