The turbid, macrotidal estuaries of the Bay of Fundy system and the coastal waters of the Gulf of Maine and Georges Bank have undergone considerable change as a result of the natural processes of sea level rise, erosion, and sedimentation since the last ice age, some 14 000 years ago. In more recent times, human activities, such as marsh reclamation, bridge and causeway construction, have caused even greater modifications to these estuarine systems, influencing their productivity, and enhancing their vulnerabihty to further natural and human modifications.

In order to understand the effects of both man-induced and natural changes on estuarine and coastal waters, the recent history of causeways constructed in the Petitcodiac, Annapolis and Avon River systems is examined. In two instances, rock-fill barriers provided for protection of reclaimed marshlands against flooding, and for highway crossing over tidal rivers, induced formation of major mudflats on the seaward side of the causeway. Stabilisation of these flats required 10-15 yr before biological communities resembled those typically found in these estuaries. The Annapolis tidal power plant may have induced greater shoreline erosion for 25-50 km upstream. These environmental changes caused by small causeways extend over much greater distances than previously forecast, indicating underestimation of the long distance effects of modifying natural tidal flow patterns.

Proposals for much larger tidal power barrages in the upper reaches of the Fundy system stimulated extensive multidisciplinary studies of the turbid waters at the head of the Bay. These indicate that tidal power development would result in significant changes in the Gulf of Maine and Georges Bank areas, more than 400 km away. Included in these effects are an increase of 15-30% in the tidal range in the Gulf of Maine, with consequent increases in shoreline flooding and erosion, and decreases in drainage potential of low-lying coastal land. Flushing rates of estuaries are expected to increase, as is the vertical mixing pattern of shallow areas in the Gulf and on Georges Bank. The latter effect may well increase productivity of some commercially important fisheries.

Many of the predictions of the environmental impact of large-scale tidal power development are based on modelling exercises involving large numbers of scientists in many disciplines. The conclusion of this review is that human modification of estuaries can have far more extensive effects in both space and time than normally recognised. Such changes often increase the vulnerability of systems to further natural and anthropogenic hazards.