Honestly, thats a waste. In 385 BC the Spartan King Agesilaus II found himself in a fairly similar situation when sieging Mantinea. But instead of wasting this beautiful chance to win the battle with minimal losses, he ordered his men to set up a blockade to block the river and redirect it towards the city's walls, tearing them down with the intense pressure of the water.

Now THAT is a great solution to a large problem.