groundwater problems and dam

[IntensiveGardener]

Hi all,
I'v got a tricky dam/groundwater question.
I got a large dam made about 10 years ago. Our place is on a slight slope with forest at the bottom and pasture on the upper part of the slope. Above us are large hills and we get plenty of runnoff and springs feeding a very small seasonal creek though our place. When it rains water rushes down the creek into the dam.
Unfortunately the dam never held water and when it filled a "spring" would appear about 50m down from it in the forrest and the dam would empty.
We have red, volcanic subsoil which is very well drained and notoriously poor for dam building.
I recently had the guy who built it come back and attempt to seal it with a huge amount of clay.
I have also tryed to improve the pastures in order to improve water penetration and saturation above the dam.
We have had rain and the creek is flowing. The water just seems to be sinking into the pasture above the dam though and has been for months.
Unfortunately now water is sinking in to the paddocks above the dam and comming out in the forrest below it!
Is there an underground stream flowing under my dam?
I was planning to get the dam rebuilt from scratch if the clay didn't work. Is this a bad idea?
thanks
ig

[gardenlen]

g'day ig,

probably is an undeground stream some where under you dam if not is is soil strata that water can flow through, now whether you can capture than in your dam is anothar question? but then dam needs to be able to hold water, we had a water table fed dam that meant the dam would never go dry. now the crunch first up anyone and i mean anyone who can drive a dozer or scraper thinks they know how to build a dam, sadly they don't! you need to talk to people who are good at their trade(a farmer will tell you as he most likely could have told you that the land was not suitable for a dam), and if your property is not suited for a dam they will tell you so then you need to look at other ways. and adding clay to try and fix the problem or to try and fend off the problem adds expense with no guarantees.

len

[SueinWA]

This is just off the top of my head, I don't really know what I'm talking about...

From what I have read and heard, volcanic soil is almost impossible for holding water. You can line it with gley, manure and clay, and all it needs is one or two small holes that gradually get larger, and the water disappears.

You may have some sort of bedrock or rock table underground, the water is sinking into your pastures, hitting the rock table (underneath the dam), and is being channeled down to your forests. If you didn't WANT the dam, this would be excellent... :lol:

A synthetic pond liner would be prohibitively expensive.

The only thing that comes to my mind is the way the folks at Gaviotas, Columbia (see book, Gaviotas: A Village to Reinvent the World by Alan Weisman - Chelsea Green, 1999 - WONDERFUL!) where the people mixed one part Portland cement with (I think) 14 parts of the local soil and made water pipes (formed over water-filled vinyl tubes - like garbage bag material). Using the same basic concept, I wonder if you could just sprinkle an estimated amount of cement over the surface of your dry dam and rake it into the top inch or so, then spray with water (or do it just before an expected rain -- which could be tricky, but effective).

It will be interesting to see what ideas others come up with.