California Riparian Systems "d0e21384"

Background

The area around Murphy Slough on the Sacramento River southwest of Chico, Butte County, is a key element in the Sacramento River Flood Control Project. It is the beginning of the overbank flow of excess water from the river to the Butte Sink and Sutter Bypass. It is also an area of considerable river dynamism, with areas of almost one section (259 hectares) each having changed sides of the river (and ultimately ownership) as late as 1921 (fig. 1).^[3]

A review of the current USDI Geological Survey 7.5' quadrangle map dated 1949, and photorevised in 1969, shows a surprising amount of vitality; with the river continuing to erode laterally, narrowing the neck of the oxbow between river mile (RM) points 187.5 and 189.5 in that 20 years. Subsequent erosion has opened the mouth of Murphy Slough to the full force of the Sacramento River (reversing the direction in which it once flowed in this channel).

The problem with this is that when an excessive amount of water is directed into Murphy Slough, some of it returns to the river, across the narrow neck to RM 187 (fig. 2). With the energy of 4.8 km. (3 mi.) of river concentrated in .4 km. (.25 mi.) of cutoff, erosion is inevitable. If the river were to cut off almost 4.8 km. of channel at this location, the headward erosion of the river bottom would lower the water surface upstream of the cutoff, reducing or eliminating entirely the overbank flow at the Chico weir site and carrying the additional water down the channel with the streambed gravels and sands.

Figure 1.
Centerline meanders of the Sacramento River near
Golden State Island, 1920 and 1969 (from Brice, 1977).

[1] Paper presented at the California Riparian Systems Conference [University of California, Davis, September 17–19, 1981].

[2] J. Fred Chaimson is Chief of the Flood Control Maintenance Branch for the State Department of Water Resources, Sacramento, California.

[3] George Carter, Manager of the M&T Ranch, Chico, California. Personal communication.

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Figure 2.
Bank lines, Sacramento River, 1980, RM 187 to RM 191
(from Sacramento River Aerial Atlas, September 1980,
Sacramento District, US Army Corps of Engineers).

There are three alternative scenarios to this.

1. The resulting increased flow and debris would stay in the river channel and overtax the levees downstream.

2. The increased velocity and flow, coupled with a rising river bottom (caused by the adjustment of the eroding river bottom upstream), would cause the river to complete eroding of its natural levees somewhere between Sidds Landing (RM 178) and Kimmelshew Bend (RM 186.5), diverting the Sacramento River into Butte Basin (Brice 1977). This would cause widespread flooding in Butte Basin, possible failure of the levees of the Sutter Bypass, and loss of usefulness of the levees of the Sacramento River from Ord Ferry to Verona.

3. The third alternative is similar to the second except that the breakout would be to the west, flooding the nearby towns of Colusa and Princeton. This alternative is considered much less likely, however.

To forestall these possibilities, in 1974–75 the US Army Corps of Engineers (CE), with the usual State participation, constructed two revetment areas on the narrow waist of the peninsula and built an embankment or plug at the cutoff on Murphy Slough. The embankment was outflanked in high water of 1978 with some damage and again in 1980 with considerable damage, including erosion of the unit three revetment at the return to the river. Using emergency funds, the CE repaired and extended the embankment at the cutoff in 1980.