I am a professional civil engineer, now retired, but I have done a good deal of hydraulic engineering, and hydrology studies. I feel I have the expertise to comment.
Beavers construct their dams for a purpose, and that purpose is to impound water to a sufficient level so that they can build shelters above the water level that are basically "dry" with air to breath, but with underwater entrances, that predators can't easily use.
Three significant factors for determining a quantity (Q) at a point downstream are: Intensity of a storm over time, runoff area and time of concentration. The last is the time it takes for a drop of water from the furthest distant point of the watershed area to reach the point downstream when it is desired to know the "Q". Longer times of concentration result in lower "Q"s.
When the beaver dam is being constructed, water is impounded, and the beavers keep making the dam higher until the desired depth is reached. While the water is being impounded, the downstream flows will be less. Once the water level is at the desired(by the beavers) level, additional water will not be impounded, and what goes in come out.
Some additional water may be impounded in high intensity storms, lessening the downstream flow. Unfortunately, dams constructed by beavers may well be damaged by additional flow in and higher ponded levels, and the outflow then is higher than would be the case without any dam at all. Outflow = inflow + flow of previously impounded water. This would result in higher flows downstream of the dam than there would be with no dam at all.
Hydrology is complex, usually with a good many assumptions or guesses. Beaver dams may, depending on the length and intensity of a given storm, lessen flooding downstream in low intensity storms and increase flooding downstream significantly in high intensity and longer storms.