Failing Banks: Lake Agassiz's Ongoing Impact EarthCache
Failing Banks: Lake Agassiz's Ongoing Impact
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The Red River flows across the flat lakebed of the ancient glacial Lake Agassiz, a massive glacial lake created from meltwaters of the Laurentide ice sheet. As this continental glacier decayed, its meltwaters formed Lake Agassiz. The river itself is very young; it began only after Lake Agassiz drained, about 9,500 years ago.
The watershed of the Red River was part of the Hudson Bay Company’s concession in North America. The Red was a key trade route for the company, and contributed to the settlement of British North America. The river was used by fur traders and settlers of the settlement eventually known as Winnipeg, Manitoba. The river gave its name to the Red River Trails, nineteenth-century oxcart trails which also supported the fur trade and settlement.
The Red River forms at Wahpeton, North Dakota flows north to Manitoba, draining into Lake Winnipeg and ultimately into Hudson Bay. The watershed of the Red River is remarkably flat; from its origin near Wahpeton, to the international border near Emerson, Manitoba its gradient is approximately approximately 1 foot per mile. Because of this, the river does not have the energy to cut a gorge. Instead it meanders across the silty bottomlands of Lake Agassiz. The river’s floodplain is the ancient lakebed, hence high water has nowhere to go, except to spread across the old lakebed in "overland flooding". Heavy precipitation in the form of snow and/or rain, has caused a number of catastrophic floods, which often are made worse by the fact that snowmelt starts in the warmer south, and waters flowing northward are often dammed or slowed by ice. These periodic floods have the effect of refilling, in part, the ancient lake.
The Red River’s banks consist of a combination of lacustrine clay-rich soils and alluvial deposits of layered clays, silts and sands. Clay soils are generally weak in strength and strongly affect riverbank stability in Winnipeg.
The alluvial layer consists of broken rocky matter, or sediment, formed from material that has been carried in suspension by the river and dropped as the velocity of the current decreases. Alluvial deposits can consist of a range of particle sizes, from boulders down through cobbles, pebbles, gravel, sand, silt, and clay. Much of the world's best farmland lies on alluvial deposits. River currents produce a sorting action, with particles of heavy material deposited first while lighter materials are washed downstream.
Lacustrine clay is commonly referred to as the lacustrine deposit or the brown clay. This type of clay was deposited in a lake environment in geologic time. The soil material consists of a clay and silt mixture. The clay is of relatively low permeability meaning that water does not readily travel through it. The nature of Lake Aggassiz resulted in the formulation of layers of lacustrine clay throughout its original basin.
Lacustrine clay soils dominate the outside bends of the Red’s riverbanks while alluvial deposits are located along the inside bends. Straight and transition sections of the riverbanks are typically composed of a combination of the two types of soils with alluvial over lacustrine soil deposits.
The lacustrine clay banks are generally weak in strength and typically have deep seated failures that can extend 200 feet in from the river's edge. These usually occur on the outside portion of the river bend.
Alluvial banks generally exhibit higher strength and are more resistant to erosion. Because of this, alluvial banks are often quite steep with sharp drops to the water. Riverbank deterioration is typically minor and limited to wave and ice erosion.
Alluvial deposits over lacustrine clay banks typically exhibit river bank movement that can range from shallow shifts to deep seated shifts. They generally occur on portions of the river that are straight and have the potential for significant riverbank failure.
As you reach the first waypoint examine the nature of the river bank failure in this particular area. What type of soil condition do you think exists here?
As you proceed south to the final waypoint, the river bank failure diminishes. What type of soil conditions exist here?
Please send the answers to these questions to me. Thanks!
If you have a digital camera please take a picture illustrating the river bank failure and feel free to include youself and/or your GPSr. Posting a picture in your log is optional.
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