Long-term At-a-Station Channel Morphological and Hydraulic Changes in Response to Decreased Sediment Supplies

Information about past stream-channel conditions is often sought but difficult to obtain. This study analyzes at-a-station hydraulic geometry (AHG) using historic stream-flow gauging data to infer channel changes over the past century. Deviations in AHG relationships are examined in the American and Bear watersheds, Sierra Nevada tributaries to the lower Sacramento Basin, California. Many channels in this region are recovering from episodic hydraulic mining sedimentation in the late 1800s. Sediment reductions were often accelerated by construction of dams upstream, although some basins received large volumes of sediment from licensed hydraulic mining in the early 20th century. Records of width, depth, W/D, velocity, stage, cross-section area, and discharge were drawn from archival U.S.G.S. stream-flow measurement records made mostly at cableways. Ordinary least squares regression removed effects of discharge variations, and regression residuals were examined for long-term trends. Most channels experienced an early period of progressive incision accompanied by lowered stages, increased depths, and decreased top widths for a given discharge. Rapid incision began at most sites before gauge initiation and stabilized during the second half of the last century. This widespread degradation represents a tremendous flux of historical alluvium from within-channel storage, especially in the early 20th century. In some cases channel changes can be identified as responses to specific watershed events such as dam construction or 20th century hydraulic mining operations. Hydraulic variables such as Froude number, specific energy, and total hydraulic head computed from the stream-flow data can reveal systematic changes in flow environments. Specific energy at some sites increased progressively with flow depths even though total hydraulic head did not due to reduced elevation head. Evaluations of these variables allow inferences about three-dimensional changes in a channel reach. For example, Froude numbers at some sites fluctuated at a high frequency in response to variations in velocity and despite long-term depth increases. The scour and fill of a large pool can be inferred from this interaction along with changes in bed material availability following dam closure. Stream-flow records are rich in information about long-term changes to channel morphology, sediment storage, and flow environments from which much can be learned about former channel conditions to enhance historical reconstructions.