Natural History Oasis de Los Osos

The climate of Los Osos is an unusual mixture of maritime Mediterranean and Continental desert regimes. Summers are very hot with occasional thunder showers typical of desert locations. Winters are typically Mediterranean with light rains and warm daytime temperatures. We do not have temperature data at Los Osos. However, because of its relatively low elevation and proximity to Palm Springs, temperature patterns are similar. Los Osos is a hot desert site where the average maximum summer temperature is 41O C (106O F). Precipitation data available for the site indicates annual precipitation averages around 333 mm (13.1 inches). However, the amount of rain Los Osos receives per year is highly variable from a maximum of 808 mm (31.8 in) to a minimum of 84 mm (3.3 in). The small amount of rain that the area receives falls primarily during the winter (Fig. 1) when temperatures are more moderate (average maximum temperature = 22O C [71O F]).
Figure 1. Precipitation patterns for the Oasis de los Osos Reserve

The San Jacinto Mountains, in which the Oasis de los Osos Reserve is found, is part of the Peninsular Ranges Province that extends from San Jacinto Peak down 900 miles to the tip of Baja California. Within the San Jacintos, location plays a major role in how an area develops. Unlike the James Reserve on the western side of the range with its cooler temperatures and higher precipitation Oasis de los Osos is a steep alluvial fan on the dry northeast side of the mountain.  Dryer conditions support sparse vegetation and result in periodic torrential rains and violent floods carving deep canyons that broaden into flood plains.  One of those canyons is where Los Osos is located and within which flows the perennial stream, Lamb Creek, tumbling over several small waterfalls as it flows onto the alluvial fan at the base of the mountain.   The varied geology of the area produces unique conditions that have resulted in diverse differences in flora and fauna of this high desert site.


Oasis de los Osos: Two native plant communities are represented at los Osos, Desert Scrub on the dry slopes and washes, and Riparian Woodland along the moist creek area. Abundant in the riparian zone are cottonwood trees, shrubby willow, wild grape, and colorful perennial herbs such as the scarlet monkey flower and the evening primrose.  Although there is a limited collection of plants from Los Osos housed at the James Reserve, if you enter Oasis de los Osos in the search page of the Consortium of California Herbaria, you can see the list of specimens from this area that are housed at other herbaria in California.


Oasis de los Osos – Los Osos provides habitat for a wide variety of resident and migratory birds, mammals, reptiles and amphibians.  However, little is known of the exact species to be found there. We are currently in the process of compiling a list for this area and welcome any authenticated records that you might have.  Contact the Assistant Director ( if you would like to share your records with us.  The stream provides habitat for the freshwater snail, Laguna Mountain Springsnail (Pyrgulopsis californiensis).