Karen Terry, University of Minnesota Extension Service, 888-241-0843

The four dimensions of stream connectivity. Credit: In Stream Corridor Restoration: Principles, Processes, and Practices, 10/98. Interagency Stream Restoration Working Group.

The last of the five components of a river ecosystem is connectivity. This relates to how ‘connected’ the river is through its system. There are four dimensions of connectivity: lateral, longitudinal, vertical, and temporal. Geomorphological processes carve floodplains of the appropriate size and shape, and rivers need to be connected laterally to those floodplains. At high flows, water spreads out into the shallows of the floodplain, which slows the water down. Slower water has less power, and so rivers with properly functioning floodplains do less damage during floods. Floodplains are also important spawning habitats for some fish species and provide unique habitat for many land animals.

Longitudinal connectivity relates to the upstream reaches of a stream being connected to the lower reaches. Barriers such as manmade dams interrupt not only fish migration but also the movement of sediment. Dams slow the water and sediment settles out of the water column, filling in the reservoir behind the dam and making the water “sediment hungry,” or capable of causing erosion, downstream of the dam.

The vertical pathway between the stream channel and the tiny openings in the sediment beneath the stream, called interstitial spaces, are important for many species of invertebrates as well as some fish species. There is still a lot to be learned about this buried part of streams, but it is thought that a large percentage of living organisms inhabit this part of the river.

The last of the four dimensions of connectivity is temporal, meaning that rivers need to be connected through time. One example of this is the seasonal flow of water through a river. In general, Minnesota’s rivers have high flows in the spring due to snow melt, and low flows in late summer. Human alterations to this flow pattern affect the temporal connectivity, which in turn affects the species that depend on certain patterns in time.

The five components of river ecosystems all play a part in determining system health. Consider northern pike, which depend on floodplain habitat for spring spawning. If the hydrology is altered so that there is no high water in the spring to reach the floodplain, or if the channel is reshaped (geomorphology) so that the floodplain no longer exists, then the pike cannot spawn.

Similarly, if the channel is blocked by a dam so that the fish cannot migrate upstream to the appropriate habitat (connectivity) or if the water quality is simply too poor to support the fish (biology), then the overall system becomes poorer. All five components need to be taken into account when assessing a river’s condition, or when a project is being done, in order to maintain a healthy, well-functioning river ecosystem.

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