First my disclaimer...I am NOT familiar with Lake Heyburn, that project or frankly much of anything in OK (aside from driving through it).
But,as a geologist...here is my thought. You mentioned that it is manmade and has cattails. If there is drainage that enters that lake (or pipes) from agriculture there is likely a high amount of nitrates from the fertilizer. Cattails are great for removing nitrates and sand is commonly used to filter out other contaminates, etc. It may be a natural filtration system to keep contaminates from the lake.
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Its not a silo then. Those areas are big. It has been a while since I have seen one but I think they were like 5 acres give or take. More then likely a valve or drain.