ECOLOGY OF THE PETRIE ISLANDS
(by Stephen Darbyshire)
The Petrie Islands are a series of alluvial deposits forming a wetland complex of elongate sedimentary ridges and backwaters characteristic of the Ottawa River below the confluence with the Gatineau River. Along with Kettle Island and the Duck Islands, Petrie Islands form a unique landform in the region chronicling the powerful geological forces which shaped the face of our country. The sand and clay sediments that make up these islands were created by the massive ice-sheet that ground and polished the continent 10,000 years ago. Crushed by the continental glaciers from the rocks further north, the sediments were carried down the Ottawa and Gatineau Rivers. The river became a major route for plants and animals moving into Canada after the glaciation. Fine sediments would have provided the closest thing to soil available on the denuded landscape. From the patterns of plant distribution we can still see the e6ects of these processes today.
The unique habitats of the river corridor contain many plant species which reached their migrational limits along the flood plain forests and shores. Many species of plants can only be found in the region along the flood plain of the Ottawa River. Some of these plants require water systems and flooding for their seeds to be dispersed. Some need the continual shifting of shoreline sediments, while others need their habitat to be inundated in the spring but dried out in the fall. All of the plant communities found on the Petrie Islands are specially adapted to extensive spring flooding.
The power of the river and its seasonal fluctuations have shaped these islands and made them a special habitat for plants and animals. The islands are almost completely flooded in the spring. Flooding of the forests on the islands is a special feature that many species are adapted to and may even require to maintain healthy populations. Continual erosion and deposition of sediments around the islands provides a renewal of shoreline habitats and an evolution of plant and animal communities. The quiet backwaters are extremely rich habitats providing shelter and abundant nutrients for a wide variety of plants and animals not found in the open river. The diverse mosaic of habitats high-energy river shores, seasonally flooded forests, quiet backwaters, sand dunes, fertile clay soils, are all formed from the same processes, unique in the region to the Ottawa River. It is this variety of habitat that supports the many plants and animals of Petrie Islands.
At one time the types of habitats and communities found on Kettle Island, the Duck Islands and Petrie Islands would have been common along much of the Ottawa River shores between Ottawa and Montreal. Today, flooding and water level control from hydroelectric projects, development, shoreline armoring and farming have transformed the flood plain habitats. The natural environments of the shores, forests and backwaters along the Ottawa River has all but disappeared and is long past regeneration, apart from a few areas such as the Petrie Islands.