Potential impacts of groundwater withdrawal and wildfires along the Big Sur River : an assessment of Oncorhynchus mykiss habitat

Oncorhynchus mykiss (steelhead trout) are a federally threatened species currently found in the Big Sur River in Monterey County. Dam construction, culverts, water diversions and sedimentation are leading causes in habitat degradation for steelhead. A recent capstone study found that near-stream groundwater wells were extracting enough water to cause a decrease in surface flow, generating motivation for this study. The Basin Complex and Indians fire of 2008 burned 84% of the Big Sur watershed creating the potential for elevated fine sediment yields. Post-wildfire sediment yield increases from background conditions to an unknown level at an unknown magnitude. The threat of continuing sedimentation generated the motivation to monitor watershed response to the 2008 fire. This study looked at post-wildfire sediment yield from the Basin Complex and Indians Fire and potential impacts of groundwater withdrawal. Discharge measurements were taken above and below a well field to determine if a loss of surface flow was occurring. There were no detectable changes in surface flow below the well field. The precision of our study was between 0.028-0.113 cms (1-4 cfs), which is substantially more than the maximum pump capacity of one well in the study reach 0.002 cms (0.058 cfs). To monitor post-wildfire sediment response, six transects were reoccupied from previous studies. Pebble counts were taken at each transect and compared to results from previous years. The Big Sur River continues to experience post-wildfire sedimentation as a result of the Basin Complex fire of 2008. Five transect sites showed a decrease in the D50 particle size. The decreases in size ranged from 45-193 mm between the 2008 and 2010 studies. Four out of six sites resurveyed have strongly impaired substrate for O. mykiss embryo survival. The lack of other impacts to the watershed suggests that the sediment fining that continues to occur is a result of the 2008 Basin Complex and Indians fire