UPDATE:
I was inspecting the site again today. Sadly, I noticed a number of fissures in the native rock that had escaped earlier notice.
Chances seem very high that my dam would hold but the rock on either side would leak like a sieve (best case) or be blown out completely (worst case).
I think I'll change plans and install a highly permeable gabion dam instead. It won't form any type of pond, but will slow the flow, capture sediment to build up the stream bed, and slowly release water downstream. It will be the first in a series of several gabion and trinchera type sediment dams along about a half mile of this arroyo.
I'll then build a large earthen dam downstream from the gabions and trincheras. The resulting pond should cover about 2.5 acres and have a maximum depth of fifteen to twenty feet. The dam itself will only need to be about 8' high. Most of the depth will be dug out. The primary spillway will be a series of culverts two feet below the top of the dam on the upstream side. They'll have a 45 degree bend so they can slop down through the dam and exit level at the natural creek bed. Emergency spillway will be a wide overflow channel around one side of the dam with a gentle slope down to the creek bed.
This larger pond, shouldn't collect much sediment, thanks to the series of upstream sediment dams.
About 50 yards downstream from the dam is the confluence of this arroyo with another, much larger, arroyo. Just beyond that, the combined arroyos enter a vertical walled canyon with a few seasonal tinajas (natural rock basins holding temporary or permanent water).
I plan to enhance the tinajas by adding some natural looking and broadly sloped masonry dams ranging from two to three feet high. I'll keep the main tinaja (8' wide x 12' long x 5' deep) topped off at all times. I'll use water from the main pond in good times, and water from my well when necessary.
I want the main tinaja to serve as a permanent water hole for wildlife and an occasional splash pool for myself. The canyon wall has a cave (8' high x 8' wide x 20' deep) with a natural rock porch and a smaller side cave. It's about halfway up the canyon wall and well above the highest water mark. It was used by prehistoric Native Americans. I plan to deck it out as a neolithic man cave. A buried water line with a hidden solar powered water pump will create a natural looking "spring" near the mouth of the cave that will trickle down to the tinaja.
I'm hoping I can stock some native minnows, catfish and perch in the large pond, but need to make sure it will hold permanent water first. I may also try stocking the tinajas with minnows, but would probably need to restock them after every major rain event. Flash floods through that canyon can be torrential and would likely carry any fish in the tinajas far downstream. Flash floods will also scour out any sediment from the tinaja basins, as long as I maintain proper slopes on the masonry dams. Otherwise, they'll just become gravel pits
Last edited by Daryl200; 04/04/16 10:15 PM.
