I have a pond that's about 70' x 40' that I would like to make deeper, it has a 4" pvc in-feed from a brook that's about 20 feet away, and an outflow back to the brook through an 18" culvert. I was planning on putting an 90 elbow on the culvert in the pond to raise the water level about 1' and so I don't have to look at the culvert heading out. When I mentioned that to a friend of mine, he said that it wouldn't necessarily reach the outflow level in the heat of summer when the brook is low. Is that so? I just assumed that the water would always come in and fill it up regardless (assuming the water level in the brook isn't lower than the in feed). The guy who built it put the inflow in the brook in one of those 36" diameter cement culverts set in vertically with a cap on top.
Any input?

It depends. Water flows downhill. If the inflow at the stream is higher than the outflow then the pond will rise and water will flow out as long as the quantity of flow is more than the leakage and evaporation from the pond.

Thanks for the reply, by eye, I believe that they are about level, but I will run a laser to see for sure.

Here are two of three ponds I have raised the level with 90 degree elbows.

raising forage pond level post

raising main pond

Water seeks its own level. The water level in the pond will never rise above the level of the creek at the inlet.