Flooding: Northwest Territories

Introduction

Ice jam flooding is the prevalent type of flooding for most of the Northwest Territories. The breakup pattern of the Mackenzie River and subsequent flooding are described below. In the mountainous areas of the western part of the territories, major summer floods result from rainstorms or glacier meltwaters in warm summers. The most extreme summer floods often exceed spring runoff flows, and sometimes exceed the maximum breakup water levels observed on area rivers and the mainstream of the Mackenzie River.

Ice Jam Floods in the Northwest Territories

In the Northwest Territories, damaging floods generally occur during the spring breakup period as a result of ice jams. Most of the territorial communities subject to flooding are located in the Mackenzie River basin, Canada's largest river basin. The Mackenzie drains parts of British Columbia, Alberta, and Saskatchewan to the south, and the Yukon and the Northwest Territories in the North.

Spring breakup starts in April in the southern tributaries of the Mackenzie River basin, generally works its way northward, and is completed in about six weeks. The Liard River triggers the breakup process on the Mackenzie and continues to influence breakup downstream, as far as the Mackenzie Delta and the Beaufort Sea. The Liard has a large spring runoff, which peaks very quickly because of its headwaters high in the mountains, steep terrain and river profile, and the absence of lakes for natural storage. It delivers large volumes of warm water to the Mackenzie, usually when that river is still completely ice covered. The large volume and high velocity of the Liard River water put tremendous pressure on the Mackenzie, causing it to break up.

In the south, towns prone to flooding are Hay River, Fort Simpson, Fort Liard, and Nahanni Butte. The hazard is the greatest at Hay River on the south shore of Great Slave Lake. A close watch on community flood conditions must be maintained each year due to the delicate balance between snowpack depth and rate of melt of accumulated winter snow, rainfall during spring snowmelt, ice deterioration before breakup, and the timing and location of ice jams in the flood-prone delta.

To the north at Norman Wells, overtopping of islands in the Mackenzie River (used as production bases for ESSO Resources Canada Ltd. oilfields) by ice and water necessitates the shutdown of wells during breakup. This is to prevent wellhead damage and oil spills. The community itself is not flood prone. High spring flows and major ice jams in the Middle Channel of the Mackenzie Delta can cause widespread flooding, affecting the community of Aklavik. As much as 95% of the delta can be covered by water in some years (e.g., 1961, 1982 and 1992).

Tuktoyaktuk Flooding

In the Northwest Territories, Tuktoyaktuk on the arctic coast experiences flooding when major Beaufort Sea storm surges occur during the late summer and fall. The timing of surges and the strength of winds are critical, as it is only in the late summer when the fetch of open water from shore to the edge of offshore ice pack is great enough for major storm surges and waves to be produced. Climate warming, sinking of land in the area, and local coastal erosion may intensify the flood hazard in the area over the next century.

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