Sediment Supply

Background and Problem

In 1996, The Central Valley Regional Water Quality Control Board designated the Fall River an impaired waterway due to increased sedimentation caused by both natural and anthropogenic sources.

The majority of sediment now resting in the Fall River likely originated from multiple sources.  In the early 1960’s Bear Creek was channelized to improve grazing. The narrower, straighter channel increased the streams ability to transport sediment downstream into the Fall River. Historic disturbances such as fires and large floods also contributed to the problem. These sources of sedimentation have been studied extensively and tens of thousands of dollars have been spent to successfully stop additional sediment from entering the river.

Unfortunately, two to four feet of fine sediment (275,000 cubic yards) still blankets much of the upper river while invasive aquatic plants continue to spread throughout the lower portion of the river.

Major sediment studies over the last 20 years include: Spencer & Ksanders (1997), Department of Water Resources (1996-1997) Tetra Tech (1998), SHN Engineers (1998-2000), and Johnson et al. (2006).

Key findings from these studies include:

1. Trends are difficult to establish prior to 1998 due to a lack of baseline data
2. Natural transport rates for Fall River are low: would take 100’s of years to flush
3. Most sediment (86%) originated from surface runoff within the watershed or from Bear Creek Meadow (bank erosion from muskrats/ cattle is a much smaller factor)
4. Dredging presents one possible solution
5. Native aquatic vegetation seems to be slowly recovering, sediment continues to flush slowly

Ultimately, the long-term impact of excessive sediment and invasive species on the fishery remains uncertain. However, anecdotal evidence from local residents, fishing guides, and landowners suggests that habitat conditions and trout populations in the Fall River continue to deteriorate.

Equally troubling is the absence of updated baseline data, which could help quantify the effects of these alarming trends on wild trout populations and the overall ecology of the Fall River.