Fast-flowing waters of the Humber River, Newfoundland, are a challenge to Atlantic salmon migrating

Physical Environment: In lotic habitats, flowing water prevents nutrient accumulations and phytoplankton blooms. The current also mixes water thoroughly, providing a well-aerated habitat of relatively uniform temperature. However, current, oxygen level, and clarity are greater in headwaters than in the lower reaches of rivers. Nutrient levels are generally less in headwaters.  Location: On all continents except Antarctica.  Plant Life: In slow-moving streams and rivers, algae and rooted plants may be present; in swifter-moving rivers, leaves from surrounding forests are the primary food source for animals.  Animal Life: Lotic habitats have a fauna completely different from that of lentic waters. Animals are adapted to stay in place despite an often-strong current. Many of the smaller organisms are flat and attach themselves to rocks to avoid being swept away. Others live on the underside of large boulders, where the current is much reduced. Fish such as trout may be present in rivers with cool temperatures, high oxygen, and clear water. In warmer, murkier waters, catfish and carp may be abundant.  Effects of Humans: Animals of lotic systems are not well adapted for low-oxygen environments and are particularly susceptible to oxygen-reducing pollutants such as sewage. Dams across rivers have prevented the passage of migratory species such as salmon.