Some species are most active at night. Very rarely are leeches which attach to humans encountered in fast moving water or riffle areas. Many are scavengers or feed on other invertebrates. They are carnivorous, feeding mostly on insects, molluscs and oligochaetes, or scavengers, feeding on dead animal matter. Silted substrates are unsuitable for leeches because they cannot attach.
Leeches are usually rare in calcium-poor waters.
The cannot tolerate high turbidity loading as well. Some species can tolerate mild pollution. Save Our Streams. Monitor's Guide to Aquatic Macroinvertebrates. Izaak Walton League of America.