Drivers of Dynamic Disconnectivity in Rivers

Dynamic Connectivity in the Landscape
Chair: Adam Ward (Indiana University)
Connectivity between different locations on the landscape is defined by the movement of water, solutes, energy, and organisms. The magnitude and persistence of connections is critical to prediction of ecological functions, many of which are mediated by hydrological stores and fluxes. In this session we consider connectivity as a spatially and temporally variable process in catchments and river systems.

\"Drivers of Dynamic Disconnectivity in Rivers
Speaker: Ellen Wohl (Colorado State University)

As the science of hydrologic connectivity has developed, there is sometimes a tendency to under-emphasize the importance of disconnectivity. Connectivity and disconnectivity can both be characterized in terms of magnitude, frequency, duration, timing, directionality, and dimension. The latter characteristic is important because features that create longitudinal disconnectivity in rivers, for example, can enhance lateral and vertical connectivity. Here, I discuss naturally occurring processes that limit longitudinal connectivity in river corridors and the effect of these longitudinal disconnections on river form and function. Examples of such processes include lateral channel movement and the associated secondary channels, avulsions and cutoffs; lateral sediment inputs from tributaries or adjacent hillslopes that create alluvial fans in the river corridor; logjams; and beaver dams. I focus on logjams and beaver dams using case studies from mountain streams in the Southern Rockies. River networks in this high-relief terrain are predominantly steep, narrow canyons with high longitudinal connectivity and limited lateral connectivity between channels and floodplains or vertical connectivity between channels-floodplains and the hyporheic zone. Wide, low-gradient valley segments scattered throughout the network provide retention zones in mountainous river networks and typically exhibit greater lateral and vertical connectivity than the intervening steep, narrow segments. The details of connectivity, however, and the magnitude of retention and the partitioning of retained water, solutes, sediment, and organic matter among alluvial storage and atmospheric emissions depend in large part on the presence of features that limit longitudinal connectivity, as such as logjams or beaver dams. Through research conducted during the past decade, we have found that channel-spanning logjams and beaver dams substantially decrease downstream transport of water, solutes, sediment, and organic matter during both base flows and snowmelt peak flows. At the same time, logjams and beaver dams substantially increase channel-floodplain and channel-hyporheic exchange and thereby increase retention and long-term (102-103 year) alluvial storage of sediment and organic carbon. One implication of these findings is that river management can be designed to limit longitudinal connectivity in ways that foster ecosystem services such as flood attenuation, reduction of downstream sediment, nitrate, and carbon fluxes, and increased habitat abundance and diversity.