The Ventura River Watershed
The Study area for this program is a section of San Antonio Creek. San Antonio Creek is a major tributary to the Ventura River.
The Ventura River watershed, with headwaters in the Santa Ynez Mountains north of the City of Ventura, has an area of 222 square miles. The river can be divided into three zones: (1) the mountainous areas of the basin; (2) the main stem of the river, from the confluence of Matilija and the North Fork of Matilija creeks to the river delta or estuary; and (3) the delta which is about 2 miles wide at the coast and extends about a mile upstream, almost to the Main Street Bridge.
The mountainous areas produce most of the sediment and water in the river. The major tributary watersheds in this zone are Matilija Creek (55 sq. miles), North Fork of Matilija Creek (16 sq. miles) and San Antonio Creek (51 sq. miles). Coyote and Santa Ana creeks (41 sq. miles) are also in this zone, but almost no runoff, stormwater or sediment from these drainages flows into the Ventura since the completion of Lake Casitas (1959, a 285 ft. earth dam that stores 254,000 acre-ft.). Matilija Creek also has a dam, built in 1948 and designed to store 7000 acre-ft. However, sedimentation (principally from the El Nino floods of 1969) have reduced its capacity to 500 acre-ft. and it is now mainly used to enhance diversions via the Robles canal.
The main stem of the Ventura River is roughly 15 miles long and is characterized by the storage and transport of sediment along a broad flood plain (generally about a half mile wide). Two major structures on the main stem govern dry-season flow, although their capacity is too low to affect stormflows. Upper most is the Robles diversion dam, which diverts Ventura River water, 2 miles below Matilija Dam, into Lake Casitas via a canal. 20 cfs (cubic ft. per sec.) of river flow must be allowed to pass the diversion, but everything above this (to the canal’s maximum capacity of 500 cfs) may be diverted. Given the porous sediments and cobles that form the river bottom, this usually insures that the river goes dry shortly below the diversion. Lower down, a concrete weir extends underground, beneath the river and Coyote Creek approximately ¼ mile above the Foster Park Bridge. While there is also a surface diversion here, the weir was designed to raise the water table to facilitate pumping to Ventura’s water treatment plant. Since the weir doesn’t fully confine the river, raising the ground water table usually insures river flow past Foster Park. Approximately a mile and a half below Foster Park, effluent from the Ojai Sewage Treatment Plant (2-3 cfs) helps maintain this year-round flow to the ocean.
The estuary, approximately 30 acres, includes a main lagoon usually separated from the ocean by a sand/cobble bar during the dry-season. The sand bar gets breached by winter storms and is slowly rebuilt in summer, fed by long-shore drift sand. In dry years the bar may not be breached at all, and it may never get established in extremely wet years. With the bar, the lagoon is mainly fresh water, without it, mainly salt and subject to tidal flushing.
The Ventura watershed is roughly 45 % mountains, 40 % foothills, and 15 % valley; 75 % is rangeland (shrub/brush) and 20 % forest (half of the catchment is within the Los Padres National Forest). While the basin is mostly undeveloped, urbanization, cattle raising and oil production dominate the coastal plain and adjacent foothills. The average annual rainfall is 20 inches and the seasonal and inter-annual variation in river runoff is extreme: mean annual flows vary from 5 to 3400 cfs (cubic ft. per sec, in other words a “wet” year can have almost 700-times more flow than a “dry” year). More than 90 % of the rainfall occurs between Nov. and April, and a majority of the annual runoff usually occurs over 3 to 7 days. The river is hydrologically “flashy” and responds within hours to storms and changes in rainfall.