Estimating the hydrographic effects of Prudhoe Bay causeway breaches usingthe before-after control-impact (BACI) analysis

A Before-After Control-Impact (BACI) analysis was used to test the effects of new breaches constructed in two Prudhoe Bay causeways on hydrographic co nditions during the open-water summer season. At West Dock, under east wind conditions, significant cross-causeway differentials in salinity and tempe rature at the surface (1 m depth or less) were observed in all eight pre-br each cases tested. In the years following construction of the breach, there were no significant cross-causeway differentials in seven of those eight c ases. At Endicott Causeway, under east wind conditions, significant cross-c auseway differentials in surface salinity and temperature were observed in all eight pre-breach cases tested. Significant cross-causeway differentials continued in all eight cases following construction of the new breach. Res ults suggest that the new breach at West Dock has successfully mitigated cr oss-causeway hydrographic differentials, and that the new breach at the End icott Causeway has had no observable effect. The possible reasons for this disparity include different hydrographic dynamics in the vicinity of each c auseway.