Some of the freshwater species found along the Colorado River and its perennial tributaries include coyote willow, arrowweed, seep willow, cottonwood seedlings and exotic tamarisk. Cladophora , a filamentous green algae, is abundant in the cool, clear river water. This algae is an important food source for aquatic invertebrates such as amphipods. Hanging gardens, seeps and springs often contain rare plants such as the white-flowering redbud tree, haplopappus and flavaria. Other freshwater species such as monkeyflower, watercress, columbine, sedges, horsetail and rushes are associated with seeps and springs. Sedges are the predominant freshwater species in stock tanks and ponds on the North Rim.