Hello, and thanks for having me in and on your site.
Question, while analizing a possibile site to build a dam for a pond of about an acre of surface area, And 4-1/2' deep, The seasonl creek has scoured the sand and gravel to bedrock, this area is about 16' in lenght. With a 6% downslope.
Should I key into the bed rock, (dense granite), and somewhat smooth, or just go on top with the clay core? This dam will also serve as a bridge with a 20' wide dirt/gravel road on top.
We get 18" of rain a year, except this year...32" to date!!!sl

Steve n Carol
If the bedrock is solid your in good shape to just cover with moist clay.
Watch for seams in the rock that could let water escape.
Another potential problem to watch for is if there is a layer of gravel or leaky material on top of the bedrock realize that water will follow that out of your pond into the banks and/or around the core.

Thanks Kent, I am glad to hear that I wont have to key into it, the bedrock. I will look for fissures too. thanks again...sl

First off, I found this site by accident today researching geothermal pond loops. Fantastic resource! There is a great forum for almost every subject if you can find it.

I would not feel comfortable placing an earthen dam on unfractured granite w/o a cuttoff trench to key in the core of the dam. W/O the cutoff trench, it would be easy for the water to move along the bedrock and destabilize the whole embankment. However, with a 4&1/2' deep pond you may not have enough hydrostatic pressure to matter that much. There are lots of variables that make giving/following advise on these matters hit or miss. FWIW, if the exposed rock is under where the dam will be, I'd "key" it in.

Good Luck!

Good post, Dirtguy.

Stick around...we could use your expertise.

Welcome aboard Dirtguy.

Ditto what the Doc says. We have lots of dirty questions. Hope you stay around.

Steve n Carol,
About 1981 2 of the ponds I constructed that year were on a solid rock ledge. Both back up about 8' of water. Neither was keyed in. Both are in good condition now.

Hi everyone! Its been 5 months since I first posted this subject, so I thought I'd give you an update.

First, thankyou for all of the imput, I value all of your(s) opinions.

Our rainfall exceeded 52" last year, I posted 36", however, that was in L.A. I later found out the 52" was recorded 1/2 mile from our construction site, (in the local mountians).

Just recently, the pond, (there is an existing dam, it's a 140 feet downstream and has been there for 20 years), has dried up. I am currently in there re-arranging the shoreline and clearing it out. I don't want to go more than 4-5 feet deep. This existing bridge/dam is now repaired, as we use that for access to our property.

I am wanting to build another, (new), dam upstream. A new dam plus the existing dam, will make two shallow ponds and give it a tiered effect. In fact, I think there are a couple of places where this can be repeated along this seasonal creek!

Back to the subject, The existing dam had to be repaired first, as this is our access. You're gonna love this...
My nieghbor, who has a great big heart, saw that the dam was overflowing over the top and across the dirt road. This was on account of the pond not being maintained the spillway culvert was plugged up and about 3 feet underwater. I come back from vacation to find a ravine 8' wide and 6 feet deep cut across our road, and another smaller one. My neighbor says, " the water was comming across the road, so I dug 2 little ditches for it to drain off in".

So with that fixed, and the water receeding, I was able to get down in there with my dozer and found that the overflow pipe was not plugged! Rather was blocked off by a big piece of a wood beam across the front of it. I pulled it out by hand, I asked my neighbor about this piece of beam, he said that he had put it there so the water would get higher! (He is a nice guy!)

Now the water has dried up except the very bottom area, I DON'T want to get stuck in the mud again, that happend too, Had to wait 4 weeks with that!

The mucking out and re-modeling the shore line will bring me a better understanding of just where to place the new dam. The original spot is still a good place, but I am going to dig around a bit more.

Another neighbor has a cashe of about 200 truck and tractor tires that are an eyesore, I was thinking about incorporating these into the new structure....any thoughts on this Idea?

Again, thanks for your opinions....sl

Yes, the tires can be used for structure. It's recommended that 20% of the pond should hold structure. Each structure should be at least apx 10 in dia. & as high as not to be unsightly protruding above the water.
There are several threads here on using old tires. Drill, saw or somehow make holes for air to escape before placing them. The holes should be large enough that they won't later clog with mud, algae ect.