Mowing the grass to the edge of the stream bed actually causes pollution, flooding, and erosion problems. It also allows for invasion of exotic plants that may take over the area.
Even small buffers are important to the water quality and local wildlife. A lawn mowed to the edge of the stream can be created into a buffer by merely not mowing a strip along the streambank. Plants will begin to grow once the area is left alone. If you would like to modify what is there, buy any plant that is native to the western Pennsylvania region, they will be good for the soil and weather.