Delta

When a stream, such as the Boquet, flows into a standing body of water like Lake Champlain, the moving water of the stream loses velocity. As the velocity of the water is reduced the amount of turbulence generated is also reduced. That in turn causes a reduction in the capacity of the water to carry suspended particles. These particles fall out of the water column and are deposited as sediments. In short, when the stream enters a lake, it deposits a load of sediments. These sediments accumulate and form a structure known as a delta.

Coming Soon - Jet plume forms lateral embankments The stream enters the lake as a sediment laden jet plume which extends out into the lake. The jet deposits some of the sediment at the end of the plume where the water slows down as it enters the lake. However, much of sediment is deposited along the sides of the plume in underwater lateral embankments. This occurs because as the sediment laden water moves into the lake its forward motion is impeded, and the water, rather than backing up, flows to the sides. As it does it flows up out of the channel and onto the underwater banks and over their sides. And as it does so, it loses velocity and deposits the sediment it is carrying. Over time this builds up an underwater embankment along the edge of the main channel. This process is similar to that in which sediments are deposited along river banks to form a levee.

Coming Soon - Lateral breakthroughs form delta shape Even without the disturbances of waves and longshore currents these underwater embankments are unstable. The jet plume will repeatedly break through weak areas in the walls of the embankments. New channels will form and sediments will be deposited along these. These new channels will form a radial pattern with the river's mouth at its apex. Over time these deposits will form the semicircular structure we know as a delta.

The Delta of the Boquet River

Bedrock geology of the river's mouth

The mouth of the Boquet was formed where two blocks of sedimentary rock faulted, then slipped, in relation to one another. To the south is a steep hill which marks the scarp formed by an upthrust block of Beekmantown dolostone of lower Ordovician age. To the north a block of Potsdam sandstone of upper Cambrian age was thrust down and tilted south so that bedrock slopes more gently into the mouth of the river (see bedrock geology of the Boquet) from that side. This sloping block continues under and forms the bottom at the river's mouth.

Near the end of the last ice age, when Lake Champlain reached something near its current level, the mouth of the Boquet would have looked very different. It was a deep bay. This bay has since been filled in by sediments and they have extended out into Lake Champlain where they form the delta.

The topographic map below shows some of the features of the delta's structure. Notice the deposits at the mouth of the river and the underwater deposits indicated by the bathymetric contours.

Delta of the Boquet River

The above water portions of the present day Boquet delta are seen in this infrared aerial photo. Although the photograph was taken in the spring when the water level in the river was still high, the photo still shows some of the features of a typical delta structure.

Infrared aerial photo of the delta of the Boquet River

The dotted white line shows the outline of the original bay mentioned previously. You can see the delta deposits which filled in the bay and extend out into the lake. The solid white arrow shows the present day channel. The broken white arrows show previous channels. Many of the old channels now contain swamps or marshes. These swamps are separated from Lake Champlain by a beach berm which has been deposited just above the lake's high water mark.

As the delta rose above the water level of Lake Champlain, what were formerly lateral embankments continued to have sediment deposited on them and thus became levees. Along the abandoned channels these levees now have trees growing on them. The current levee along the right (south) bank of the present day river is composed of coarse sand and supports only a few small trees and woody shrubs; along the left (north) bank there is less sand and young trees are growing.

Some delta features are above the surface of the water. But the real delta is the deposits of sediment under the surface of the water. Please continue on to the underwater delta.

 


 
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