Stormwater carries mud, waste, and nutrients to lakes. It can get this bad (photo from Madison, WI)
Once in the lakes, nutrients feed the growth of toxic algae.
Erosion and stormwater are one of the largest sources of nutrients to ponds on Cape Cod. Stormwater runoff causes the erosion, and also caries nutrient-rich debris to the pond--dog waste, human waste, soil, leaves, and garbage.
Nutrients in turn cause toxic algae blooms, and the growth of aquatic weeds that reduce the quality of swimming and other uses. Once nutrients get into the lake, they are extremely difficult to eliminate. The pond is degraded forever. Alum treatment can improve the lake, but it's expensive and has to be repeated periodically.
Erosion around ponds isn't recognized as a problem, because such erosion is normal for the seacoast. Most people are familiar with ocean beaches, but the ponds are less visited. So people think erosion around ponds is normal. In fact, they probably enjoy the enlarged beaches caused by erosion and trampling of shore vegetation. But the enlarged beach eventually degrades water quality--the very reason they came to the beach.