I have some rather startling news and a warning for people living along the Mojave River — especially residents of Spring Valley Lake, Merrill Gardens, Jess Ranch, the Equestrian Center and Desert Knolls in general. If you experience even a moderate earthquake, get out and head to higher ground. You are in danger of a catastrophic flood.
This warning comes from me and two geology instructors retired from Victor Valley College. Although not on the fault maps, the Cleghorn Fault runs beneath Cedar Springs Dam at Lake Silverwood and down the length of the Mojave River to just past the Upper Narrows. And the Cleghorn Fault crosses the San Andreas Rift Zone. When the San Andreas — the local portion, which has not moved since 1857 — moves, there is a good chance the Cleghorn Fault will shift, also. And if it shifts — well, you get the idea.
I believe the entire north-south section of the Mojave River is "graben" — a down-fault. Hesperia and Victorville are on the "horst" — the up-fault — the section that hasn't moved. Most of Apply Valley lies in the graben, the lower part of the fault, and is therefore subject to flooding. All of Jess Ranch and Merrill Gardens, both retirement communities, also lie within the flood plain of the Mojave River, as does the lower section of Spring Valley Lake. I have at least two reasons for believing the Cleghorn Fault runs the north-south section of the Mojave River.
First, a river generally runs through a lower zone, and although it is generally
assumed that the Upper Narrows formation was split by water flow alone, I believe it's actually a fracture zone. That is, actually a fracture along the Cleghorn Fault Zone. Past the Upper Narrows, the fault continues approximately north. Note the difference between the foot of Quartzite Mountain and Stoddard Ridge, the hills east of Stoddard Wells Road. That whole section looks like a graben. Past the Upper Narrows, the Mojave River turns westward and no longer flows along the fault zone.
Second, although the Mojave Water Agency maintains that Hesperia, Victorville and Apple Valley are on the same aquifer, there is an enormous difference in water quality and mineral content from the west (Hesperia and Victorville) side of the Mojave River and the east (Apple Valley) side; this is indicative of a fracture in the aquifer zone, in other words, a fault — the Cleghorn Fault.
The lower campus of Victor Valley College, Spring Valley Lake and the Mojave Narrows Park are also in the flood plain of the Mojave River, as is the Kemper Campbell Ranch. Note the "bluffs" in Spring Valley Lake and Victor Valley College and the higher ridge in Victorville, the eastern portion of "Hospital Hill."