Summary


Quantification of seepage characteristics of the lower 20 river miles of the Clackamas River, from Estacada to Oregon City, Oregon, was derived from individual streamflow measurements and decades of streamflow data. Interpretation of gains and losses in streamflow based on historical measurements was aided by analysis of time-of-travel of water pulses released from River Mill Dam. Many site-to-site differences in streamflow were relatively small. 


Gains and losses of streamflow of the lower Clackamas River, after accounting for tributary inflows and withdrawals, are discernible on mile-by-mile and reach scales, and can be attributed to the geomorphic setting of the basin. The loss in streamflow between Estacada and Barton, confirmed by measurements in May 1992 and September 2006, is attributable to infiltration into recently reworked streambed sediments. The streamflow gain between Barton and Carver was confirmed by measurements in September 1998 and September 2006. The gain is attributable to a basin-scale constriction in the stream channel that forces subsurface flow to the active stream channel through shallow stream deposits overlying relatively impermeable sandstone. Downstream of Carver, the stream intersects permeable Pleistocene flood deposits and loses streamflow between Carver and Clackamas, based on streamflow measurements made in September 2006. 


Many gains and losses in the Clackamas River from Estacada to Clackamas and Oregon City are less than the uncertainty, indicating that if gains or losses actually are occurring, they are small relative to the total streamflow. On the basis of August–September daily mean streamflow, and after accounting for tributary inflows and withdrawals, only in some years were gains from Estacada to sites at Clackamas and Oregon City greater than the uncertainty, and no losses exceeded the uncertainty. Generally, these results were confirmed by streamflow measurements made in August 1987 and September 2006. 


Gains in streamflow in the lower Clackamas River during summer are smaller than gains in the upper basin. On a per-unit-area basis, gains in streamflow were greatest in the High Cascades geomorphic area, diminished in the Western Cascades area, and less than the uncertainty in the part of the Clackamas River draining the Willamette Lowland area, downstream from Estacada. 


First posted November 28, 2011