Pilarcitos Integrated Watershed Management Plan - San Mateo ...

Pilarcitos Integrated Watershed Management Plan
for rearing. If the pond does not continue to operate in the future, rearing habitat would most
likely be classified as “Fair.”
5.2.4.5 Apanolio Flashboard Dam and Apron Removal, Downstream of the BFI Property Line
This site includes an existing flashboard dam and apron and an unstable (incising) channel grade.
Channel downcutting below the concrete apron at an unused flashboard dam has produced a
nearly impassable barrier to migrating adult steelhead (Alley 2007a). The apron should be
removed to prevent further downcutting, and the channel grade should be stabilized.
Project Rationale
This project would remove a fish passage barrier, reduce sediment production from further
downcutting, and improve downstream habitat if the pond continues to operate. If the
downstream Barrier 1 were modified, this project would allow steelhead access to an additional
one mile of potential steelhead habitat of relatively low quality before encountering the first of
four barriers on BFI Property (Alley 2007a). As shown in Figures 2 and 3, spawning habitat has
been rated as “Fair” upstream of this site, and rearing habitat has been rated as “Poor”.
5.2.4.6 Barrier 3 on Apanolio Creek
The recently installed boulder weirs at Barrier 3 on Apanolio Creek may require maintenance
based upon the observations of boulder transport at similar structures on Arroyo Leon and Mills
Creek in 2007 (Alley 2007b).
Project Rationale
Observations of channel widths upstream suggest that the reconstructed channel is likely to widen
after storm flows to more than five feet. Modifications might include approaches to stabilize the
existing boulder weirs, with particular attention to the stability of the weir boulders and boulders
keyed into the streambanks. The weirs should be re-evaluated after large peak flows (e.g.,
approximately bankfull stage) to assess upstream fish passage opportunities, particularly at the
lowermost weir.
Maintenance of the recently installed weirs will ensure steelhead passage past the new bridge
after downstream barriers have been removed. A relatively short stretch of stream is made
available by this project because the flashboard dam apron upstream (see 5.2.8) has become a
substantial passage barrier due to downcutting below the apron in the last 10 years. As shown in
Figures 2 and 3, spawning habitat has been rated as “Poor-Fair” immediately upstream of this
site, and rearing habitat has been rated as “Fair.”
Identified Priority Projects 39 October, 2008