My pond has been running full for several weeks with frequent small flows thru the overflow pipe that draws off the top. Downstream of the dam I have 4 small settling pools. They drop out any sediments (mostly non-pond runoffs) and sometimes dry out between rains. When fed normal surface water, the pools take on a green hue and sometimes get a thin green scum when slow/stagnant.

This weekend, pond was full and more rain coming so I opened the drain line. It draws from 8-15' down. One can posit that it's under the thermocline- it's cool. Running the drain, overflow stopped. After a day the settling pools had all black water. It developed an orange foam downstream of the first rocky drop where oxygen might be added. After I closed the drain, the first pool had a shiny film on it that looked almost like oil. There is a distinct unpleasant odor, perhaps a sulfide? I was in the creek and the dark water and odor was observed quite a way downstream from where the branch dumps into the creek.

Is this typical anoxic water? For now I only have forage fish, but seeing all that doesn't breed confidence that large fish would want to be there in the future.