Calleguas Creek


The major river in Zone 3 is Calleguas Creek with a watershed area of approximately 341 square miles. Other channels in the zone are relatively small coastal drainages that outflow to Mugu Lagoon or the Pacific Ocean. Major tributaries to Calleguas Creek include Revolon Slough, Conejo Creek, Arroyo Santa Rosa, Arroyo Conejo, Arroyo Las Posas, and Arroyo Simi. Virtually the entire watershed is in Ventura County. Historically, Calleguas Creek and its tributaries were intermittent and flowed seasonally from its headwaters near the City of Simi Valley onto the Oxnard Plain. Due to development, Calleguas Creek is now primarily a perennial stream predominantly fed continuously by treated wastewater flows, with secondary surface flows originating from rising groundwater, agricultural and urban runoff, and periodic stormwater flows (Calleguas Creek Watershed Management Plan, 2002).

Zone 3 incorporates three of the five of the supervisor districts for Ventura County. The majority of the zone falls within Supervisor District 4, with the rest of the zone falling within Supervisor Districts 2 and 3. The superintendent for Zone 3 is Ernie Bravo.

Summers in the Calleguas Creek watershed are relatively warm and dry and winters are mild and wet. Frosts are rare in the coastal region, but common in the inland valleys and mountains. The types of storms that may occur in the basin are general winter storms, thunderstorms, and tropical cyclones. Rainfall occurring during the winter season, which extends from November through March, is approximately 85 percent of the rainfall total. Mean annual precipitation varies from about 13 inches on the Oxnard Plain to 14 inches in the inland valleys, with a maximum of 20 inches in the higher elevations. The wettest rainfall year with recorded data occurred in 1941, when 38 inches of rain fell. The driest rainfall year was 1894, when only 3.3 inches of rain were measured.

The Calleguas Creek topography is typified by rugged, mountainous terrain in the northern and eastern portions. In the south and west, the landforms consist of rolling hills, alluvial valleys, and coastal floodplains. Approximately 42 percent of the watershed has slopes greater than 20 percent, with another 46 percent of the watershed having slopes of less than 10 percent. The watershed is part of the Transverse Range geomorphic province of California. The geologic structures in the watershed are a result of different sequences of sandstones, shales, conglomerates, and siltstones lain down during the different epochs, with basaltic flows and intrusives also occurring. During the most recent epoch, the Quartenary, marine clays, sands, and gravels were deposited, including the Fox Canyon member that is an important groundwater aquifer in the area. Stream, terrace, alluvial fan, and swamp deposits of silts, sands, and gravel, up to 200 feet thick, were placed in the valleys and creeks in the watershed.

A high degree of tectonic activity exists in the area. In the 1994 Northridge earthquake, Oat Mountain was uplifted 18 inches. As a result, the erosion processes have not kept pace with uplift, but occur in dramatic forms such as landslides and soil slips. Except on the major floodplains, natural channels can be deeply incised. The incised channels are generally unstable, leading to erosion and presenting a complex sediment management problem to consider when designing flood control improvements.

The Calleguas Creek watershed contains the Bard Lake water supply reservoir. More than 20 District debris basins of varying sizes are located in this zone to capture sediment before it is delivered to the Calleguas Creek system. Some of the largest District basins include Sycamore Canyon, Las Llajas, and Runkle Canyon basins. Areas of major flooding in the zone include the tributaries to the Arroyo Simi in the city of Simi Valley, and Calleguas Creek downstream of Hwy 101.

Calleguas Creek Hydrology Model Results Webpage

Calleguas Creek Plans and Studies