Take Out Beach is a great pet-friendly spot to go to let your dog splash around in the Colorado River. This sandy beach has a few calm inlets that as perfect for letting your dog cool off, and it's right along Highway 128, so it's easy to access. As its name suggests, many boaters on the Colorado River take out here, so make sure to keep an eye on your pup.
Safety Note: Do not let your dog swim in the river when it's running high (typically late May-July).