CONNECTIVITY IN HYDROGEOMORPHIC SYSTEMS

Studies of hydrogeomorphic systems are increasingly carried out by interdisciplinary research groups to provide an improved scientific basis for management and predication of ecological and physical processes. Though watersheds are a common unit of study, the heterogeneity of geomorphic processes in different valley reaches coupled with variation in connectivity between reaches can diminish the predictive capability of a watershed approach. We describe connectivity and controls on connectivity to hydrologic, sediment, and channel processes based on two studies. Research in alpine riparian systems in Nevada was carried out to understand hydrologic connectivity and geomorphic responses; and in a lowland, agricultural system in KwaZulu-Natal, South Africa to study sediment and nutrient transport. In all areas, channel slope systems were mapped by geomorphic traits.

In Nevada, groundwater flow is near continuous between reaches, but zones and rates of recharge vary with season, channel substrate, and channel connectivity. Typical annual surface flow patterns show reaches of perennial flow in headwaters regions with intermittent flow in reaches of thick sediment cover. Lack of surface water flow between reaches is because it was hydrologically limited (e.g. insufficient flow for a losing stream) or geomorphically limited (no channel). The long-term influence of basins with high connectivity is reflected by deeply incised, flood dominated channels. Conversely, basins with low connectivity are less incised and, in places, have thick accumulations of sediment that support groundwater dependent ecosystems. An increase in connectivity can cause a decrease in groundwater recharge, and an increase in surface flow and sediment transport. A drainage ditch that was constructed in a previously unchannelized alluvial valley in KwaZulu-Natal resulted in a 5 fold increase in sedimentation rates in a downstream wetland.

The connectivity between valley reaches is dynamic, specific to a process, varies spatially and temporally, and can be altered by nature or people. Recognition of the connectivity of watersheds can provide models to foster understanding and guide management on topics ranging from channel sensitivity to water quality to riparian ecosystems to the interaction of groundwater and stream water.