Oral reports by four local residents provided qualitative evidence for erosion during storm flows in Robinson Creek. One resident recalled that channel depth increased during the 1986 flood (personal communication, Troy Passmore, Mendocino County Water Agency, 2005). A second resident who has lived near Robinson Creek since 1933 noticed a deepening of about a meter in the past 20 years in both Anderson Creek and Robinson Creek; he has not seen overbank
flow during floods such as occurred signaling pathway during water years 1937, 1956, 1965, 1983 (Navarro River Resource Center, 2006). A third resident born in Boonville in 1936 said his house is ∼5.5–6.1 m above the creek but remembers when it was ∼4.6 m with banks that were not as steep. He said banks have been sloughing since ∼1965 and he has lost ∼9–12 m of land from bank erosion during high flows. He also mentioned that willows were uprooted during such floods (Navarro River Resource Center, 2006). A fourth resident living Cobimetinib clinical trial along Robinson Creek (upstream of Mountain View Road) for more than 35 years said she did not notice incision, but that widening began in the past decade (Navarro River Resource Center, 2006). These recollections suggest that over the past 80 years, incision and erosion have been spatially variable active processes
during floods—but that incision in Robinson Creek had also occurred prior to the 1930s. Comparison of thalweg elevations in repetitive channel cross sections measured from Adenosine bridges provided quantitative evidence to aid in determining the timing of recent incision. First, the elevation of Anderson Creek’s thalweg near the confluence of the two creeks, that is effectively the baselevel for Robinson Creek, has lowered in the past decades. Repetitive cross sections surveyed across Anderson Creek at the recently replaced Hwy 128 Bridge (∼90 m upstream of the confluence) shows a thalweg elevation lowering of almost 1.0 m at an average
rate of ∼0.026 m/yr during the 38 year period between 1960 and 1998 (personal communication, Mendocino County Water Agency, 2004). Second, several of the bridges crossing Robinson Creek within the study reach are incised such that bridge footings are exposed (Fig. 5). For example, the current Fairgrounds site was built on the location of a mill that was active through the 1950s. The present bridge is estimated to have been constructed in the 1960s when the site was acquired, with bridge repairs recorded in 1969–1971 (Jim Brown, personal communication, Mendocino County Fairgrounds Manager, 2013). Field measurements in 2008 indicated that the bridge footing has undercut ∼0.9 m. These estimates suggest that incision occurred at an average rate of ∼0.019–0.024 m/yr (between 2008 and 1960/1971, respectively), similar in magnitude to the estimate of baselevel lowering in Anderson Creek.