The Lake of Two Mountains is part of a river delta (an area of exceptions, as river splitting is normal - but note that individual riverways are temporary, deltas are rapidly shifting areas of water flow), and it also is part of an artificially regulated system, although I haven't been able to find any indication that the lake itself is artificial. It is a shallow lake and could perhaps arguably be described as a wide section of river. Looking at it on Google Maps, it appears that all these different outflows join up again rather quickly, meaning that what one actually has is a set of islands breaking up the outflow.