Southern Subbasin Overview
The Southern Subbasin includes the watershed area along the SLR River from Rice Canyon, approximately one mile east of Interstate 15 (RM 19) to just upstream of the Escondido Canal diversion dam (RM 40) (Figure 1). Stream elevations range from near sea level in the lower mainstem to approximately 3,400 feet in the headwaters of the tributaries. The Southern Subbasin occupies approximately a quarter of the total basin at 134 square miles. This assessment area is mostly rural with the exception of the expanding city of Valley Center. Pauma Valley and portions of the communities of Bonsall and Pala are also located within its boundaries. The Rincon Indian Reservation and to a lesser extent, the Pala Band of Luiseño Indians and San Pasqual Indian Reservations, reside within the subbasin. The majority of the subbasin is held in larger private parcels managed for agricultural crop production.
With the exception of the SLR River canyon (RM 37-39.5), the majority of the Southern Subbasin provides little suitable habitat for steelhead as the mainstem and many of the tributaries are dry for the majority of the year with seasonal flows limited to the precipitous periods of the winter and early spring. The SLR River canyon, located in the eastern portion of the subbasin, provides perennial flows with potentially cool water temperatures and deep pools. The mainstem SLR River was most likely used mainly as a migration corridor for adult steelhead to more extensive spawning and rearing habitat located in the Northern Subbasin tributaries and possibly in the mainstem within the SLR River canyon. Juvenile steelhead may have used the mainstem for rearing habitat during their downstream migration to the estuary and thus the ocean. Located in the SLR River canyon at RM 39.5 (approximately ½ mile downstream of the Escondido Canal diversion dam; Luiseño place name “Kye”), is a 50 foot high natural waterfall (Figure 10). This waterfall is broken up into a series of steps; with the largest lowermost step approximately 13 feet, and a narrow steeped crevasse above the first step extending to the top of the waterfall (M. Capelli, personal communication 2010). Considering the altered flow regime with the majority of the stream water being diverted approximately ½ mile upstream at the Escondido Canal diversion, steelhead are unlikely to navigate through this feature.
Historical evidence of the presence of steelhead in this subbasin is mainly attributed to anecdotal accounts from local tribal elders who spoke of annual runs (USFWS 1998) and the presence of steelhead “…coming up from Oceanside” (Soto 2008). There is limited direct documentation of steelhead in the mainstem or in its tributaries. This is due, in part, to the lack of coordinated survey efforts by CDFG or any other organization. Prior to the 2007-2008 CDFG watershed assessment surveys, CDFG had not surveyed the SLR River in the Southern Subbasin since the 1940s. The CDFG performed this reconnaissance level survey effort in 1946. This survey contained the last documented report of non-hatchery rainbow/steelhead trout in the river (near Pala) within the Southern Subbasin. It is unknown whether these fish had become resident rainbow trout or if they could potentially migrate downstream to the ocean. Another reconnaissance level survey was performed by a fisheries biologist in 1975, but it provided little quantitative or qualitative information about the SLR River within the Southern Subbasin.
For a brief period in the 1990s rainbow trout were stocked in the SLR River within the Wilderness Gardens Preserve boundaries by schools that were provided fish from the CDFG’s Mojave Hatchery. Water quality monitoring and sampling of aquatic insects indicated at the time of the releases that conditions were favorable for trout survival. Since the termination of the trout release program information collected in the river within the subbasin has been mostly limited to water quality monitoring performed by the Pala Band of Mission Indians.