WHAT IS RIVER RESTORATION?

River managers often pursue restoration, commonly agreed to be a return to a pre-disturbance physical state as socially and technologically practical as possible. The pre-disturbance state, however, is usually both unknown to the body of scientific literature and can not be determined from available data. Further, selection of a specific disturbance can be subjective: rivers are dynamic and constantly "disturbed" by both human and non-human causes.

In the absence of a defined pre-disturbance condition, generation of restoration targets often relies on modern reference rivers. We used this strategy to consider hydrologic management targets in Connecticut rivers identified by The Nature Conservancy (TNC). We used TNC's Indicators of Hydrologic Alteration (IHA) to calculate annual hydrologic parameters for a water supply river and a reference river over the same period of record. IHA results were tested for significance using the Signed-Rank Test. While many results appeared sensible, baseflow in particular was surprising: managed river median = 0.07 cfs/mi²; reference = 0.04 cfs/mi² (p<0.001, n=24 yr). Based on this analysis, the recommendation for hydrologic restoration of the managed river is to greatly decrease baseflow, a critical parameter driving instream ecology.

Both the reference and managed watersheds, however, have suffered similar disturbances of colonization and later land use modifications. For instance, landowners improved drainage by removing in-stream woody debris, and deforestation removed the replacement wood load. Loss of instream woody debris can decrease baseflows by a factor of 1.5 or more, as shown by others. Deforestation might offset this impact, with a water yield increase of 40% found by others. Extirpation of beaver, however, likely resulted in an even greater decrease in baseflows than loss of woody debris. Hydrology can also be impacted by increased impervious area, loss of wetlands, etc. Given multiple unknown disturbances, a recommendation based solely on analysis of modern rivers can be unsound.

Without definition of a river restoration target, it becomes impossible to generate scientifically-based recommendations. Some strategies for generation of a protocol to define pre-disturbance conditions in southern New England are presented for discussion.