The oral histories suggest that Robinson Creek banks were already high prior to the 1930s. To constrain our estimate of the timing of the initiation of incision, we used proxy data including measurement of

incision in relation to undercut riparian tree roots, and surmised that incision began after these riparian trees established after the early 1810s but before the 1930s, consistent with the timing of incision estimated selleckchem from the oral histories. Although this time range generally coincides with the initiation of intensive land use disturbance in Anderson Valley, it leaves uncertainty about whether the incision began in the decades just before, or after the initiation of significant land use disturbances in Robinson Creek watershed. One plausible scenario is that initiation of intensive sheep grazing in the watershed (that peaked in the 1880s) increased runoff to channels. The increased discharge to sediment load ratio could have initiated incision and increased the transport capacity of storm flows. Subsequent landuses that likely increased sediment supply, such as agriculture on the valley

floor and logging on hillslopes, would have decreased the discharge to sediment load ratio, but apparently not enough to reverse the effective routing www.selleckchem.com/products/Y-27632.html of sediment through the Robinson Creek watershed, despite development of new sediment sources such as eroding channel banks or inputs from eroding tributaries. Local fluctuations in river bed elevation may result from translation or dispersion of sediment waves Nicholas et al., 1995, McLean and Church, 1999 and Sutherland et al., 2002). Similar fluvial responses have occurred in Urease Anderson Creek, the effective baselevel for Robinson Creek, as both Creeks drain an area of Anderson Valley with similar land

use histories. The presence of several apparent knickzones in Robinson Creek upstream of the confluence with Anderson Creek suggests that incision is caused at least in part by headcut migration that occurs because of the downstream baselevel lowering in Anderson Creek, currently occurring at a rate of ∼0.026/yr. Using this rate to project back through time requires assuming that incision occurred at a similar rate over the 145 years between ∼1860 when grazing began and 2005 when the profile was first surveyed in the study reach. Using this average rate suggests that baselevel lowering could potentially account for ∼3.8 m of the total bank height, with 1.0–4.2 m of bank height remaining at the upstream and downstream end of the study reach, respectively, likely related to other factors such as historical landuse changes that modified upstream watershed hydrology and sediment supply or to local structures intended to limit bank erosion, that progressively channelize the study reach and prevent widening.