Water diversions can significantly alter a river’s ecology in a number of ways. Amongst the most pronounced are that you end up with (see the illustration above):
• a smaller river, (stream width)
• a narrower floodplain, and (width of floodplain)
• and an incised channel (elevation of water height).
Irrigation canals, drainage ditches, and water-impoundment features (e.g., ponds) are common throughout the lower mainstem Jocko River floodplain. The construction of these artificial features often results in localized changes in floodplain hydrology and leads to an increase in fish entrainment (fish getting trapped in canals), spread of noxious and non-native plant species, and poor habitat for other aquatic species. Management and restoration plans for protected properties will prescribe techniques to remove or modify existing water diversions and/or impoundments as an initial step to restore floodplain hydrology.
Irrigation withdrawals can also cause channel sinuosity (how much a river winds or meanders) to decrease because channels become destabilized from carrying an increased sediment load, while the ability to transport that load has decreased. If sinuosity decreases, the slope of the stream channel increases, which increases the stream's energy and the damage done by the stream during floods.
Irrigation withdrawals can cause water temperatures to increase and this can seriously impact fish like bull trout and cutthroat that need cold water. Other impacts, like land clearing and grazing practices in the floodplain, can also impact water temperature.
Many riparian trees and shrubs require depositional surfaces where their tiny seeds can germinate without competition from other plants. Reduced flows caused by irrigation withdrawals can reduce the number of depositional features, thereby reducing the numbers of tree and shrub stands that become established over a period of several years.
Effects of drought years may be more pronounced due to irrigation withdrawals; this can cause higher mortality in young trees whose roots have not yet reached their maximum depth. It can also result in loss of wetlands.
Other impacts from irrigation practices include fish entrapment in unscreened diversions, unstable diversion points, and water quality problems associated with the irrigation return flows.
Irrigation diversions can also prevent migratory forms of bull trout and westslope cutthroat trout from moving upstream into spawning habitats.