Need suggestions for clearing up silty pond

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Backstory:

Pond is 60' x 75' and about 4' at the deepest. It was originally created by simply digging down with a dozer, and pushing the excess dirt on the downhill slope to create a berm. No liner or other improvements were added to the bottom - much of which was hard sandstone.

We added 10 each, 3-in long Koi and Goldfish, never to be seen again in the muddy/turbid waters. We also added 10 ducks. I installed to aerators and a fountain that kept the water moving, but no filtration. In the subsequent two years, the water never cleared enough to see the fish unless they were feeding naturally (we did not feed them). The ducks, digging in the berm, looked to be causing its failure - so they were rehomed. Within 24 hours, hundreds of fish came to the surface, gasping for air - turning the pond orange/white. We had no clue any had survived until then.

We tested the water and all values came out perfect - nitrates/nitrites, pH, hardness, chlorine, etc - I cannot recall all the variables, but they were all exactly where they should be. I added a few more aerators (total of 5, 10-inch stones) and an IBC tote with those 1" spikey palls. A 4500gph pump with suction at the center of the pond pushed water into the IBC, with the return being a gravity release over large pond stones to prevent turbulence of the pond silt.

About two weeks ago, I added two gallons of flocculant that I had sitting around from an earlier project. Within 24 hours, the pond was crystal clear for the first time ever. It looked like a swimming pool. I could see the fish playing, all the way to the bottom/center. It was amazing. It lasted about 24 hours.

I don't want to add more flocculant as I'm sure dosing the fish is far from ideal. However, I was given a directive by my wife to do what needed to be done to get the pond clear like that all the time.

My plan:

At this point, I'm almost certain that the cause of my turbidity is from the clay-like silty soil that is being kicked up by the carp burrowing and playing at the bottom/edges. Since I have no liner, the bottom is nothing more than soft, mushy powdery soil. Walking in it, my feet sink several inches into what feels like mud.

My plan is to pump about 4k gallons into an old portable pool, an additional 1200 into extra totes I have on hand, and then drain the rest of the pond over my bluff. Once the water is low enough to "catch" the fish, I'll transfer them to the pool and keep them there until the work on the pond is complete.

Once the pond has had a chance to thoroughly dry, I'll have the bottom of the pond scrapped to remove all the scum, firm up the berms, and then spread to about 40 tons of crush and run gravel across the entire water-bearing area of the pond. Other neighbors used the same gravel when they originally constructed their ponds (no liners), and they've always had clear water. I'm hoping that's the missing ingredient in my pond.

Any suggestions?
 
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Well, looks like I'm having a conversation with myself... that's ok, I'll take input as I go...

Over the last few weeks, I was able to remove all the fish and put them in my 16' diameter 40" deep (4k gallon) pool. I added two large aerator stones and a circulating pump, pumping through a 275 gal IBC tote with bio-balls. So far, no casualties. If I had to guess, there's probably 300 fish in there. Three over 24" long, but most about 8-10" long.

I then had the pond scraped down to the sandstone, and spent several days washing the sandstone with a 200gpm pump. It came remarkably clean.

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The sandstone was buried under two feet of sludge and silt - probably 2-3 years of duck/fish poop and whatever mud/clay material was left from the original excavation of the pond.

The plan is to lay large shot-rock along the water's edge to keep the dirt berms from sluffing back into the water. Originally I considered putting down a bed of crush and run gravel, but considering the awesome natural rock features along the bottom and three sides, I think it would be a shame to conceal all of that. Besides, the gravel would likely never really settle on the hard sandstone - it's like concrete.

There is one area where the rock isn't present, and I intend on digging that as deep as the excavator will reach - 14' below the sandstone bottom. This should provide additional water reserves (for, God-forbid, fire use), and added depth for minimizing water temp swings.

Any thoughts on whether hard pan sandstone like can be used as the sole bottom for a pond?
 
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I have no experience or advice but I'm interested in how this goes. Do you know what kind of substrate you're likely to find at the bottom of that additional 4 foot hole?
 
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Probably just more sandstone. We've only dug down a couple feet below the surrounding sandstone cap to provide a sump for the rain water to collect. It was that water that I used to wash down the main areas over the last few days. Once it dries out a bit more, I'll get the excavator to start digging. But if it's like anything else around here, I'll hit a hard pan sandstone. Just hoping it's deep enough to give me the extra volume I want/need.
 

YShahar

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Wow! That's quite a project. I don't think many people here have unlined ponds, which is why people weren't offering suggestions. I'm just intrigued that the sandstone itself holds water like that. I would expect it to be porous, and gradually drain the water down through it. Definitely keep posting here; I'm curious to see how the project progresses!
 
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I'm with @YShahar - you're way behind the typical garden pond with this project! But it's going to be interesting to watch. I hope all this work will solve your issues- what a gorgeous pond you'll have.

The ducks may be adding to the silt level in the pond - they aren't the cleanest birds and do enjoy pooping in the water.
 

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