That *might* happen, but you have to keep in mind that river flow rates vary depending on the season and thus evaporation and precipitation. Smaller channels would only fill up when you had a lot of rain over a short period. If you look at alpine rivers, especially when they hit the lowlands, they tend to start threading in their streams and creating vast gravel beds with the main streams shifting around pretty swiftly - even a single massive precipitation event (MPE ) can radically change the look of such a river - not to mention that it can simply redirect during floods (e.g. lots of rivers on the Po plain have shifted around quite a bit from Roman times to now).