Open File Report 1972-01

Open File Report 1972-01

OFR 1972-01

Geology of the Peace-Athabasca River Delta Region, Alberta

Author(s) Bayrock, L.A. Root, J.D. Date 1972-05-01

The Peace-Athabasca Delta is underlain by Canadian Shield granites and gneisses, Athabasca Sandstone, and Devonian limestone and gypsum. The configuration of the bedrock surface underlying the Delta varies with the type of formation present. The bedrock surface of areas underlain by Canadian Shield rocks is knobbly with local relief of 200 feet. The bedrock surface of areas underlain by other formations is nearly flat. Large preglacial valleys dissect the bedrock and these valleys have been overdeepened by glacial action. Towards the end of the recession of the continental icesheet large preglacial lakes covered the Delta region and thick deposits of inwash and glaciolacustrine sediments were laid down.

The Peace-Athabasca Delta began to form about 10,000 years ago and it now forms a composite, inland delta of the bird's-foot type. The Peace, Athabasca and Birch Deltas occupy an area of 1475 square miles and, at present, extend their total area 1 square mile every 13 years. The Athabasca and Birch rivers are actively extending their delta areas but the Athabasca Delta accounts for the greater proportion by far. The Athabasca Delta has reached a point of development (the present distributaries are overextended); where migration of the active lobe is imminent.

The Peace Delta is now in an old stage of development because the Peace River bypasses the Delta. The Delta receives sediment only during high flood stage of the Peace River so that area extension from sediment deposition is small. The Peace Delta is now effectively inactive and will remain so with the upstream regulation of water levels.

A series of barrier islands have formed (as a result of wave action in Lake Athabasca) which extend from Fort Chipewyan to Old Fort Point.

Most of the deposits on the surface of the delta are silts and clays deposited during flood stages. These silts and clays are suitable for road construction. Subsidence due to sediment compaction is evident on the margins of the Peace, Athabasca and Birch Deltas. Wave action has modified portions of the Delta, which receive little sedimentation.

Two types of lakes exist in the Peace-Athabasca Delta: lakes connected to open water and lakes not connected. The connected or open lakes raise and lower their water level in response to water level fluctuations in the major lakes. The closed lakes enlarge or decrease in area in response to rainfall and or floods.

Erosion features in the Mamawi Lake bed show that the Peace-Athabasca Delta has experienced very low water levels in the past.

No drastic changes will occur in the Peace Delta but the Athabasca Delta is a dynamic feature and large, abrupt changes in its configuration are expected in the very near future.