Several rivers such as the Kongakut River in Alaska and the Firth River in Yukon empty into the Beaufort. The major river to flow into the sea is the Mackenzie, Canada's longest. The coastal shelf area is rather narrow, especially near and east of Point Barrow in the Alaskan part of the sea. It becomes wider near the delta of the Mackenzie River. It never gets bigger than . Near the coast, the depths are shallower than . The quickly get bigger northwards up to a few kilometers. There are many small islands in the sea and in the delta of the Mackenzie River. A few larger ones are west of the Mackenzie River, such as Herschel Island ( off the shore, and Barter Island ( from the coast. The coasts are low, with the maximum elevations between . The soil is frozen all year around forming permafrost. Only the top few tens centimeters thaw in summer. Buildings have to be above ground on wooden poles.