Assessing geomorphic change along the Trinity River downstream from Lewiston Dam, California, 1980-2011

Abstract

The Trinity River Restoration Program, one of the nation’s largest adaptively managed river restoration programs, requires periodic assessment to determine the effectiveness of management actions in restoring channel dynamics and habitat features. This study documents riparian and channel changes along an intensively managed 65-kilometer reach of the Trinity River in California, downstream from Lewiston Dam. The two primary periods of interest, from 1980 to 2001 and from 2001 to 2011, are separated by a shift in restoration activities mandated by the U.S. Department of the Interior December 2000 Record of Decision. The post-2001 restoration strategy increased managed-flow releases, gravel augmentation, watershed restoration, and mechanical channel rehabilitation. We assessed the nature and extent of geomorphic change and a series of ecological performance measures (channel complexity, shoreline length, and channel–floodplain connectivity) by using a series of maps digitized from available rectified orthophotography acquired during low-flow conditions in 1980, 1997, 2001, 2006, 2009, and 2011. Lateral changes in riparian and channel features were used to quantify alluvial processes, and a review of existing streamflow, sediment, and restoration records was used to assess causal mechanisms. During the study period, natural bank erosion and mechanical rehabilitation of channel margins converted riparian features to channel features and expanded the active-channel area. The primary period of bank erosion and expansion of the active channel was from 1980 to 1997. Subsequent bar accretion from 1997 to 2001, followed by slightly greater bar scour from 2001 to 2006, took place primarily in the central and lower reaches of the study area, downstream of Indian Creek. In comparison, post‑2006 bank and bar changes were spatially limited to reaches that had sufficient local transport capacity or sediment supply supported by gravel augmentation, mechanical channel rehabilitation, and tributary contributions. The highest rates of change in the areal extents of channel and riparian features were observed during the pre‑2001 period, which was longer and relatively wetter than the post-2001 period. A series of tributary floods in 1997, 1998, and 2006 increased channel complexity and floodplain connectivity. During the post-2006 period, managed-flow releases, in the absence of tributary flooding, combined with gravel augmentation and mechanical restoration, caused localized increases in sediment supply and transport capacity that led to smaller, but measurable, increases in channel complexity and floodplain connectivity in the upper river near Lewiston Dam. Extensive pre-2001 channel widening and the muted geomorphic response of channel rehabilitation sites to post-2001 managed flows highlight the need for continued monitoring and assessment of the magnitude, duration, and timing of prescriptive flows and associated geomorphic responses.