Northern California’s Trinity River was impacted by gold mining from the mid-1800s to the mid-1900s, adding sediments from upslope and overturning floodplains. Before the river could recover through natural flooding, dams were constructed to export water for agriculture, reducing river transport capacity. Tributaries continue to supply unimpeded flow and sediment to the river resulting in sediment accretion and erosion at tributary confluences. Understanding delta dynamics and how they distribute potential ecological services for juvenile salmonids may benefit channel restoration elsewhere.
Sub-decadal aerial photography from 1944-2001, then annually 2005 to present, documents historical changes to the river landscape from the mining era, through dam-induced channel simplification, to reintroduction of channel complexity via restoration. This imagery has long been used to analyze planimetric change of the Trinity River. New technologies such as Structure from Motion (SfM) Photogrammetry and Terrestrial Laser Scanning (TLS) capture 3D point clouds of sites with survey-grade accuracy and can be repeated for change analysis.
This study examines three tributary deltas to determine: (1) do we gain substantial information from sub-annual SfM and TLS surveys over annual aerial photography, and (2) is the topographic diversity that can provide ecological services to salmonids distributed uniformly at tributary deltas?
Results/Conclusions
Remote sensing through aerial photography suggests a gradual progression toward greater geomorphic complexity at tributary deltas, with a tendency to form low-water alcoves toward the downstream end of the deltas.
TLS and SfM surveys starting in spring of 2015 revealed greater topographic change than aerial photography alone. Winter storm events aggraded the deltas and caused the channel to migrate up to 7m into the mainstem river channel compared to 2014 aerial photography. A relatively large spring restoration flow scoured much of this and deposited sediments downstream. The subsequent aerial photography did not capture the magnitude of winter aggradation and subsequent scour.
Topographic differences were found between sediment deposits at the tributary confluence versus those at the downstream tail of the delta. Ephemeral channels form primarily along the back-side of the delta connected to the alcoves seen in aerial photography. Such alcoves and side channels are thought to provide good habitat for juvenile salmonids by providing appropriate water depths and velocities, along with food production. The topographic variability provided by tributary deltas with ephemeral channels and alcoves can serve as a model for restoring juvenile salmonid habitats elsewhere in dammed river systems.