USGS Finds Groundwater Major Source of Streamflow in Upper Klamath Basin

Groundwater discharging to streams through springs and seeps is a major source of streamflow in the upper Klamath Basin, helping to sustain flow during the dry months of late summer and fall, the U.S. Geological Survey (USGS) announced on April 26.

Although the connection between groundwater and surface water has long been understood and documented in earlier reports, a new USGS report is the first to provide quantitative estimates of groundwater discharge to streams over the entire upper basin.

The sources of groundwater in the upper Klamath Basin are rain and snowmelt that infiltrate through the soil into the aquifer system at an estimated rate of about two million acre-feet per year. Most of this recharge occurs in the Cascade Range and uplands in the interior and eastern parts of the basin. Groundwater generally flows toward the interior basins and stream valleys, and most of the flow eventually discharges to streams.

According to Marshall Gannett, lead author of the report, the lava flows and other volcanic deposits that dominate the geology of the basin are very permeable and form productive aquifers. Groundwater can discharge from these aquifers as springs or seepage to streams where geologic and hydrologic conditions permit.

"Although streamflow in many western basins diminishes or ceases entirely during dry parts of the year, groundwater-fed streams in the upper Klamath Basin have substantial flows year-round," Gannett noted. "Groundwater also tends to reduce the effects of drought, because aquifers provide carryover storage from previous years. However, after several dry years, spring flow starts to decrease."

As a result of the connection between groundwater and streams, groundwater pumping can affect streamflow. Pumping water from an aquifer in one location affects groundwater flow elsewhere. Springs are one of the places where flow can be diminished as a result of pumping.

The report, developed in cooperation with the Oregon Water Resources Department, describes how the water table fluctuates in response to drought cycles and pumping. Ken Lite, from the Oregon Water Resources Department and a coauthor of the report, stated that "groundwater levels historically have been stable in the upper Klamath Basin, moving up and down mostly in response to drought cycles. Pumping has caused some declines in the past, but these generally have been modest and restricted to relatively small areas."

Lite added, however, that increases in pumping in recent years have caused water level declines of as much as 10 feet to 15 feet in some deep wells in the Klamath Valley and northern Tule Lake subbasin. It is not known how long it will take the water table to recover once wet climate conditions return and pumping is reduced.

The report, "Ground-Water Hydrology of the Upper Klamath Basin, Oregon and California," can be viewed online at http://pubs.usgs.gov/sir/2007/5050.

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