Floodplains are lands bordering a river that can become inundated during flooding. They are considered part of the river channel during high flows.
Floodplains provide many critical functions in the watershed. They are natural water storage reservoirs, acting as a sponge to absorb floodwaters, reducing flood heights downstream, and slowly releasing the water as the flood recedes. Many terrestrial species use floodplains year-round throughout their lifecycle, but floodplains also provide critical habitat for many fish when water levels are high. Floodplains can also capture sediment and filter water, cooling down river temperatures.
Distribution of floodplains
Floodplain forests, often dominated by deciduous trees, are found along lower elevation valleys of the MFWW, especially downstream of the dams.
Threats to upland habitat in the MFWW
Floodplain Connectivity: Dams regulate 80% of the land within the MFWW. Regulation of discharge by the dams has affected peak flows and the amount of time that floodplains are underwater. Changes in the timing and magnitude of floods has resulted in loss of floodplain habitats, affecting the establishment of pioneer plant species such as cottonwoods and reducing the formation of side channel habitat, which provide habitat for Oregon chub, Chinook salmon, cutthroat trout and bull trout. Other significant impacts to floodplain connectivity include development in floodplains and channelization of rivers.
Council goals for floodplains:
- restore floodplain connectivity for flood mitigation and habitat enhancement
- support the release of flow pulses from dams
Projects involving floodplains:
- Educational slideshow
- Geomorphic Study of the Middle Fork - coming soon
- USACE Floodplain Feasibility Study - coming soon