Why are rivers wavy?

Answer 1

Bodies of water such as lakes, rivers and oceans collect in suitable geological formations - natural basins and reservoirs - and move within these formations in response to geological factors, tidal factors, forces such as rainfall, floods, man-made alterations and so on and, of course, to the force of gravity. Initially these bodies of water are contained within their original catchment area, obeying the confines of that area. As other factors kick in the water gradually forces its way through weaker barriers and around stronger barriers. So a man-made concrete canal or dam will retain its original shape for a very long time, while natural formations such as rivers will follow the landscape as necessary: the landscape shapes them, and this is why they are never straight, nor have straight edges. Even a fairly featurless landscape will not confine naturally-forming bodies of water in straight lines and in any case the force of water will eventually alter the plainest of plains.