There are a number of poisonous snakes in California deserts with the most common one I have seen being the sidewinder rattlesnake. I would be wary of letting my dogs off-leash in the desert mainly because a dog may not see the snake or, if it does, challenge it. I went camping in mid-October and woke up in the morning to find a sidewinder coiled up directly under my porch steps. The dogs had already bounded out and luckily did not notice it laying in the wind blown carpet. Sidewinders are usually out at night but other rattlesnakes, such as diamondbacks, are out in the day. In Anza-Borrego SP dogs are not allowed on or off trails, on leash or not.

