Introduction


The Clackamas River (fig. 1) provides drinking water for about 300,000 people and supports native fish populations as well as numerous other species (Clackamas River Water Providers, 2011). The Clackamas River currently provides downstream communities with a reliable source of municipal and drinking water while meeting in-stream requirements for fish. However, demand for water for municipal, irrigation, recreational, and in-stream uses in the basin is increasing. 


The sources of water for streams that drain the western slopes of the Cascade Range in northwestern Oregon, including the Clackamas River, are snowmelt and rain that reach the streams by way of surface runoff or discharge from the groundwater system after percolating through soils and fractured rock (Piper, 1942; Lee and Risley, 2002). Characteristics of the connections between the groundwater and surface-water systems in the Clackamas River basin have not been well defined, and to evaluate the quantity and quality of water available to water providers, an improved understanding of those connections is necessary. The study described herein was an assessment of streamflow gains and losses in the Clackamas River, which are a result of groundwater contributions to the river (gains) and groundwater recharge from the river (losses). The study was a cooperative effort of the U.S. Geological Survey (USGS) and the Clackamas Watershed Management Group (Clackamas River Water Providers and Clackamas County Water Environment Services).


Purpose and Scope


The purpose of this report is to present an assessment of the interaction of the stream and groundwater system of the Clackamas River basin by analysis of gains and losses in streamflow. Data available for this analysis consists of streamflow measurements made during previous hydrologic investigations on the Clackamas River in 1987, 1992, 1998, and 2001; continuous records of streamflow from 1963 through 1983 and 2001 through 2006; and a detailed suite of streamflow measurements made during baseflow conditions 
in 2006. 


Study Area


The primary focus area of this study is the lower Clackamas River from Estacada to its confluence with the Willamette River in Oregon City. This segment of the river is about 20 mi long and flows from east to west, from an elevation of about 300 ft at Estacada to near sea level at the confluence with the Willamette River in Oregon City (fig. 1) (Carpenter, 2003). Stream width at low flows range from about 200 to 300 ft. The river flows primarily in a single channel, although the stream channel is braided in places. From Estacada to Carver, the stream flows over quaternary alluvium (Madin, 1994). The stream narrows at Carver in a gap between hills and flows over Pleistocene volcanic sandstone overlain by thin alluvial deposits. Downstream of Carver the stream again intersects alluvial deposits and is bounded on the south by Troutdale Formation gravels and on the north by Pleistocene Clackamas River terrace deposits (Madin, 1990). 


Land use follows a gradient, from primarily forest near Estacada to a mix of agricultural and rural residential land uses toward the west. Land use adjacent to the stream in the lowest several miles of the Clackamas River is urban and industrial. The towns of Estacada, Carver, Clackamas, and Oregon City are situated along the river. This report also includes discussion of streamflow in the upper Clackamas River from Timothy Lake to Estacada by way of the Oak Grove Fork of the Clackamas River, a distance of about 42 river miles.


Precipitation in the basin falls as rain and snow. The headwater area of the Clackamas River, at an elevation of about 7,000 ft, accumulates snow each year, but by late summer, the basin is generally snow free. The period of primary interest for this study is the summer base-flow period, when precipitation is minimal and streamflow is sustained primarily by groundwater discharge. For the combined months of August and September, the average precipitation at Estacada during 1971–2000 was 3.4 in., about 6 percent of the annual total (PRISM Group, 2008).


Streamflow of the Clackamas River is regulated by several reservoirs. During the summer low-flow season, streamflow in the lower river is regulated by River Mill Dam, a hydroelectric facility at Estacada (fig. 1). Long-duration fluctuations during summer are due to water releases at Timothy Lake, a storage reservoir in the upper Clackamas River basin. 


First posted November 28, 2011