FROM DESERTS TO RAIN FORESTS: EROSION REGIMES AND THE RECOVERY OF PACIFIC SALMON

Pacific Salmon occupy rivers in diverse environments, ranging from deserts to rainforests. Erosion regimes and sediment retention mechanisms vary by ecoregion, leading to regional patterns in types of erosion impacts to salmon habitat (e.g., channel aggradation vs. incision). Recovery of salmon habitats depends in part on recovery of erosion regimes and sediment retention mechanisms, and recovery time varies by ecoregion. In loess deposits and erosive volcanic rocks of the western desert ecoregion, fine sediments dominated historical sediment supplies, and sedge meadows retained those fine sediments (silt and clay) on valley floors. Over the past 3000 years valley floors aggraded from 1 to 22 m, creating large shallow aquifers that provided consistent streamflows and stable temperatures for salmon. Since the late 1800s, grazing and agriculture have caused channel incision through valley fills, which drained shallow aquifers, reduced stream flows, and increased stream temperatures. In the wet coastal forest ecoregion, mass wasting dominated erosion processes historically, and logging practices over the past 150 years have increased supply of both coarse and fine sediments. Channel responses vary, with channels typically aggrading in basins dominated by competent rocks, but degrading in soft sedimentary rocks where wood debris removal has reduced retention of relatively fine-grained sediments (sand and fine gravel). Recovery of habitats in aggraded channels of the coastal forests can be relatively short, as time to export aggraded sediment is generally on the order of 10¹ years. Recovery time for incised channels in coastal forests is on the order of 10² years because wood debris that once retained finer sediments (sand and small gravel) will take many decades to grow and recruit to channels. In western deserts, sedge meadows can recover in a few years, but time to aggrade incised channels to their historical elevations can be on the order of 10² to 10³ years.