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Excerpts from the Executive Summary

 

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The discovery of gold in the mid-1800s led to widespread channel and riparian degradation. The channel bed was completely altered, and hillsides were washed into the river in search of gold, elevating in-channel sediment supply and storage. However, the streamflow regime was unregulated until the early 1960s, allowing for deposition and erosion processes of excess sediment to occur. Portions of the channel margin were vegetated with mature riparian hardwoods such as alders and willows, while other portions were unvegetated mobile features (i.e., gravel bars) that provided habitat for juvenile salmonids and other aquatic fauna when inundated. Local overbank deposition within willows established near the low water edge created linear mounds of fine sediment parallel to flow along channel margins, termed berms (Figure E-1). Although degraded, the Trinity River was still capable of reworking the channel bed and banks under the natural flow regime.

 

Further degradations occurred after World War II, when the Trinity River Division (TRD) of the Central Valley Project was constructed. The TRD includes three dams: Trinity, Lewiston and Whiskeytown dams. Trinity and Lewiston Dams are located on the Trinity River. Trinity Dam is upstream and impounds up to 2.4 million acre-ft of water. Water released from Trinity Dam passes into Lewiston Lake and through Lewiston Dam into the Trinity River. Lewiston Dam regulates Trinity River streamflows.

 

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Two hypotheses regarding detrimental riparian encroachment were developed during the Integrated Assessment Plan (IAP). The first encroachment hypothesis (H1) is from the TRFE and posits that detrimental riparian encroachment is a long-term threat that must be managed because riparian hardwoods can establish along the low-water edge during a series of consecutive dry years, when post-TRD flows and sediment transport are insufficient to scour plants away. The second hypothesis (H2) came about shortly after ROD streamflows were first implemented in 2005 and arose out of disagreement between some Program participants regarding the underlying mechanisms of riparian encroachment and berm formation, as well as the current state of riparian encroachment within the Restoration Reach. H2 posits that detrimental riparian encroachment is not  a long-term threat because the post-ROD streamflow and sediment regime are sufficient to scour away initiating riparian hardwoods (Section 7). Results of this synthesis report showed that there is insufficient evidence to accept or reject either encroachment hypothesis (Section 9.3). The short post-ROD period, combined with sporadic coarse sediment augmentation and modest changes in the physical system, led to inconclusive results.

 

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