Globally, rivers have become the most altered of ecosystems, chiefly due to pollution, water withdrawals, and dams that have modified their former function, and led to large and unforeseen impacts, particularly for fish populations. Extensive research is directed at studying impacts of dams because they sever migration routes and change the physical template (flow, temperature, and sediment and organic loads), and by extension, influence vital rates of fish populations such as growth, survival, movement and recruitment. Prior to introduction of nonnative fishes and network of dams, the humpback chub (Gila cypha, chub) was broadly distributed throughout the Colorado River (mainstem). Since then, chub have declined over their entire historical range and are now restricted to six populations, a factor that led to it being Federally listed as an endangered species. The largest of these chub populations is found in Grand Canyon and is isolated from other upstream populations by Glen Canyon Dam (Dam). Over 90% of this population resides within the Little Colorado River (LCR) and mainstem in regions adjacent to the confluence. The remainder is broadly distributed in small aggregations throughout the ecosystem. Cold water temperatures from the Dam has largely impeded the growth and spawning of chub in the mainstem. Luckily, chub spawn and rear young successfully in the seasonally warm and saline waters of the LCR, though survival of some juveniles (< 200 mm total length) that disperse into the mainstem varies among years.
