Till has low porosity and permeability. Welcome to the wetlands. Here the stream is spread over a larger area. This area is a marsh. A marsh contains a variety of small plants, sedges and grasses.
Hydrophilic plants are specially suited to survive in soils that are always wet.
Cat tails and Phragmites are competing species of hydrophilic plants. The narrow-leaved variety is more tolerant of pollution. The cattails here are narrow-leaved. Often wetlands that are polluted have the Common Reed known as Phragmites instead of catails. Wetlands act as a giant filter sorbing the pollution in the water that flows through it.