Chalone Creek Restoration
Many of these problems became evident during a 40-year flood event in 1998. In areas where the creek was confined by the road, significant scour and extremely high sediment loads were observed. Erosion monitoring markers, placed 1 meter deep into the stream bed, were all eroded out. Downstream of this section, the channel aggraded with massive deposits of sand and gravel, channel sinuosity increased, the channel widened, and significant bank erosion occurred. Without the “pressure relief valve” of the floodplain, this altered stream behavior will continue to occur. The changes in stream behavior are partly to blame for the destruction of a road bridge in 1998, and the subsequent $1.5 million cost of rebuilding. The increased channel scour and fill destroyed nearly all of the summer pools required for year-round inhabitation by the California Red-legged frog. The rapid channel bank erosion also destroyed much of the vegetation and shelter in the riparian area, reducing the habitat value tremendously. In this section of creek, approximately 100 trees were uprooted from the banks.
Flooding of alluvial channels is often dynamic since the bed is more easily mobilized. However, there is evidence that this dynamism exceeds historical ranges. Many of the cut banks were comprised of well developed soils and fines, that have been in place for 1000-10,000 years. Air photos dating back to the 1950’s show a trend towards widening channels and loss of riparian cover. Park documents of past flood damage, flood “control” projects that entailed blading of the streambed, and historic ground photos support this trend.
Chalone Creek is the principal drainage of the Pinnacles Formation, an uplifted block of erodible volcanics. Considering its tectonic setting, rapid increase in stream order (bifurcation ratio), and flashy watershed slopes, finding an analogy or reference stream to Chalone Creek is difficult. However, the visual and quantitative data existing for large flood events in 1983, 1995, and 1998 show an evolutionary trajectory. Geomorphic evidence and large live oak root crowns have been used to track changes in bankfull height, historic floodplain surface, and floodplain/channel relationships. Additionally, some reaches show less dynamism than others, and provide an additional reference comparison.