Farmland is great! It grows our food and (especially in minnesota) develops fuel for BioFuel so we can use it to make energy. But everyone knows that if you have too much water, plants don't grow. This means farmers are constantly working to keep water from building up on their feilds. By making the land more impervious (not allowing fluid to pass through), it can stop farmland from flooding.
The issue with this is that it's important for water to filter through the ground before it gets to our waterways! The ground filters out chemicals and pollutants and it also slows down the flow of water. When there's more impervious land, the water moves into the rivers at a faster rate, causing more erosion of the river basin and increasing turbidity (how cloudy water is). Dirt and other debris also get pulled into the river since the water isn't filtering through the ground, this causes sediment to build up in the bottom of river making it shallower and shallower and more mucky. Imagine being a fish trying to breathe in mucky water!