Large warmwater rivers are complex ecosystems and often contain numerous species and habitats. We loosely define a large river as having a drainage area greater than 50,000 km2 and a stream order great than six. Further, these rivers typically have mean discharges greater than 1,500 m3/s. Channel patterns are highly variable among and within large rivers, generally forming a meandering pattern. Currently, many large rivers are confined by bank stabilization and are characterized by a straight channel because of anthropogenic alterations. Further, most large rivers in North America have altered hydrographs because of main-stem dams or dams within the drainage area. Large rivers that have been modified to reduce meandering and flooding present challenges for deployment and operation of fish sampling gear.

Water temperature in large rivers is highly variable across North America and varies longitudinally within a river. For example, the Missouri River originates in the Rocky Mountains and terminates at the confluence with the Mississippi River near St. Louis, Missouri. Thus, mean water temperature in the headwaters is considerably different than the confluence. As a result, we defined warmwater by the dominant fish assemblage found in the river. Most fish species can be categorized into broad categories, such as coldwater, coolwater, or warmwater, based on water temperature in the natural environments where they occur. We define warmwater rivers as rivers that were naturally void of coldwater fishes such as trouts, salmons, and ciscoes. This is appropriate because it disregards zoogeographic boundaries that may limit the inclusion of rivers in the warmwater category. For example, the Colorado River is in a different zoogeographic subdivision than the Ohio River (Moyle and Cech 2000), but both rivers contain warmwater fish assemblages.