Side Channel Formation, Evolution, and Persistence

Side channels are flow paths connected to the main channel at discharges less than or equal to bankfull flow. These flow paths increase the hydraulic and geomorphic complexity of river systems, which provides opportunities for aquatic habitat by increasing the shoreline length and the variability of depth and velocity. Channel margins and areas of lower velocity are important habitat features in rivers where flow and sediment alterations have created a more uniform main channel with reduced seasonal inundation of floodplains and off-channel habitats. Therefore, constructed side channels are a common habitat restoration technique to increase the ecological value of river systems.

We analyze historical data for three river systems in the western United States to better understand how side channels form, how they evolve, and how long they persist. The river systems are the Middle Rio Grande in New Mexico, the Sacramento River in California, and the Trinity River in California. These rivers include reaches with different bed material types (sand to gravel), runoff regimes (snowmelt to monsoon), and sediment supply (a large supply of fine sediment to a low supply of coarse sediment). Side channels are identified and classified using aerial imagery between 1935 and 2012 with a time series of at least five different years for each river. Classification types consider if the side channel was likely created by erosion or deposition through processes such as lateral channel migration, obstructions or medial bars, or channel avulsion. Once formed, side channels experience evolutionary cycles such as the transition from a flow-through channel to a backwater area to a terrestrial forest. Side channels may persist for decades or may be disconnected after a single high flow event. Failure to persist is caused by an elevation change in the side channel (deposition) or main channel (incision), migration of the main channel away from the side channel, or avulsion of the main channel into the side channel.

We correlate the number of side channels, classification types, life cycle stages, and longevity to hydrology, sediment, and reach characteristics. For example, the number of side channels has decreased over time as peak flows have decreased and river systems have been altered by human activities. Classification type and life cycle evolution depend on factors such as channel confinement and lateral migration. Information from this study will help restoration designers select appropriate sites for side channels, optimize designs for a given site, and establish expectations for maintenance needs and longevity.