Sespe Creek Geomorphic Assessment

The Sespe Creek watershed is one of the most pristine and geographically remote areas in southern California.  Located in the Transverse Mountain Ranges of Ventura County, the watershed has steep hillslopes mantled with shallow soils and shrub vegetation, and is drained by steep, coarse sediment-bearing tributaries.  In 2006, the Day Fire burned a third of the watershed, stripping the vegetation cover and altering soil properties on the hillslopes.  Concerns have been raised by citizens living downstream over the potential increase in fire-induced sedimentation near the City of Fillmore levee, which could decrease flood protection levels during winter storms. 

Stillwater Sciences, in collaboration with RBF Consulting and Aqua Terra, is working with Ventura County Watershed Protection District (VCWPD) to produce a comprehensive assessment of the channel’s response to the Day Fire.  The study aims to understand the fire-related effects on hydrology, hydraulics, and sedimentation in the watershed, to allow for appropriate flood protection policies in the lower reach.

Geomorphic Investigation

We performed a fluvial morphology and sedimentation analysis to assess watershed conditions from a historical (baseline) and contemporary (post-fire) perspective.  Our study entailed a comprehensive literature review of fire history and fire effects to sediment production in the region, a characterization of watershed-wide hillslope processes and determination of sediment yields, and a characterization of sediment transport and channel morphodynamics in Sespe Creek.  Applying a GIS-based methodology we developed for the neighboring Santa Paula Creek watershed, we determined estimates of average annual hillslope sediment production and delivery to the channel network.  We extended this method to provide an estimate for post-fire sediment yields using field- and literature-based data.  Field data, historical photos, and topography were used to analyze the past 80 years of channel evolution in the lower reach, allowing us to identify areas of relatively rapid channel migration, scour, or sedimentation.  Our assessment of channel bed sediment character along the majority of the creek will be used by RBF Consulting in their sediment transport modeling effort to predict future channel changes in response to the Day Fire event.

Overview

Location:
Sespe Creek, Ventura County, California

Client:
Ventura County Watershed Protection District
RBF Consulting

Project Lead(s)

Glen Leverich
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Work Products

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Draft Geomorphic Processes Report
12-1-08 Public Meeting Presentation

IMAGE

Vegetation cover in the upper watershed was denuded by the Day Fire event, which in turn has increased fine sediment production on the hillslopes and delivery to the mainstem Sespe Creek.

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