If the face of the dam is lowered by about three feet flow would move upstream during periods of low summer river flow. During high spring flows, when fish are coming upstream to spawn high tides would stem the flow of the river enough to allow upstream passage for all species.
Although we would no longer see water levels in the pond rise to the height of the photo below.
We would have a front row seat to the rebirth of a tidal marsh.
It would probably look something like this depending on tide and seasonal water levels.
Interestingly, the water level will not drop much lower than this whether the dam is there or not. At this level the dam is no longer the hydrologic control. The narrow opening at the top of the photo is the remnants of the original dam. You can see the berm and boards which were part of it to the right, and there are old submerged timbers beneath the water in the center. At these water levels this old structure and another elevation rise in front of the dam create the impoundment upstream of the dam.
The photo below shows the restriction and elevation rise directly in front of the dam which we mentioned above. During periods of low flow such as these the dam is no longer a factor in controlling water levels.