The construction of the lake was completed around the time period of 1942 to 1943, after the filming of the serial Perils of Nyoka, although the creek bed was there from the beginning of the ranch.
The locations for the Flynn film were already set before Ray purchased the ranch property. In fact, at the same time that he was cleaning up the ranch for use as a movie location, the Flynn production was already set for lensing at Bidwell Park in Chico, California. On top of that, just prior to the production's departure to Chico, one of the crew wrote a memo that stated (paraphrased) "why wasn't the film being shot entirely at Lake Sherwood instead of traveling up to Chico".
For pickup shots of Sherwood Forest after the production returned to Southern California Lake Sherwood was utilized, as well as the Warner ranch facility in Calabasas, the old Busch Gardens in Pasadena, and one other location that escapes my mind at the moment.