Chapter 3 - Natural Resources - Marine Corps Base Camp Pendleton

MCB Camp Pendleton, California
3.1.4.1. Watersheds
Mountain ranges divide the Base into seven watersheds or drainage areas (Figure 3-2) and
(Table 3-1): San Mateo, San Onofre, Coastal Drainage, Las Flores, Aliso, Santa Margarita,
and San Luis Rey. The Las Flores watershed is often divided into two sub-watersheds, the
Las Pulgas Canyon and Piedra de Lumbre Creek. The Coastal Drainage is located north
and south of Las Flores Creek watershed. The San Luis Rey River drains the southern
portion of the Base into the ocean just south of the Base.
The largest drainage, the Santa Margarita, is 742 square miles (474,880 ac). Over 90% of
this watershed is located off Base. De Luz Creek is an important tributary to the Santa
Margarita, and provides a significant quantity of water from off Base. Aliso Creek, Coastal
Drainage (Horno Creek, French Creek, Cocklebur Creek, Hidden Creek), Las Flores (with
tributaries Las Pulgas and Piedra de Lumbre), and San Onofre (with its Jardine Canyon
tributary) are watersheds that are completely on Camp Pendleton and drain into the Pacific
Ocean. San Mateo Creek drainage (with tributaries Cristianitos and Talega), includes areas
of the Cleveland National Forest, San Onofre State Park, the City of San Clemente, and
other private lands.
The two largest watersheds on Base, Santa Margarita and San Mateo, form broad alluvial
plains as they approach the Pacific Ocean. As the streams reach the sea, sloughs or
estuarine lagoons form due to sand bars or narrow tidal barriers. These impound low
stream flows, but are breached during high-flows caused by storm events and normal tidal
fluctuation. Based on water quality data, measurements, and observations for the Santa
Margarita River Estuary, incoming tides are generally able to top the sandbar once or twice
a day, while a small natural channel in the sandbar allows water to drain back out when the
water level of the outgoing tide falls below the elevation of sand accumulated on the
sandbar (Entingh pers. comm. 2009). The sandbar blockages subsequently reform by
sedimentation and normal wave action. Gage height data for the SMR estuary indicates that
the sandbar at the mouth of the estuary only isolates the estuary from the Ocean for
approximately 25% of the time annually (Leedshill-Herkenhoff 1989).
The three largest estuaries on the Base are situated at the mouths of the Santa Margarita,
Las Flores, and San Mateo streams. Review of USGS data for the period October 2007 –
September 2008, indicates that the Santa Margarita River, Aliso Creek, and Hidden Creek
estuaries are tidally influenced while the San Onofre and San Mateo estuaries are
predominately isolated from the Ocean. The Cockleburrr and Las Flores estuaries are
somewhere in-between (Entingh pers. comm. 2009).
3-4 Chapter 3 – Natural Resources