Diverting water is an ancient tactic. You can flood an area or dry it out, depending on need. It takes time, but during a siege, time (and idle army hands) is plentiful! So you put them to work damming up this flow, diverting that flow, and voila! The city has no water! OR, make the water flow past/along the walls. The sandy soil of the region slowly but surely washes away, leaving nothing to support those big bad walls. All it takes is one area to collapse and POW youre IN