At a river mouth, sediments are often deposited due to the slowing of the current reducing the carrying capacity of the water. This is the situation as the confluence of the Niagara River and Lake Ontario at Niagara-On-The-Lake becomes quite shallow, with hundreds of feet of prominent sandbars not far from the shore.
Waves break on the sandbars that form on the lake floor which were formed due to currents and wave action. The sand bars can shift with different storm, current and swell patterns which means that waves don’t break in the same spot every time. This can pose a danger to boaters who try to navigate the bar to enter the river. As an example, as recently as September 1, 2021 a sailboat ran aground on the bar and required rescue.
New Landform Creation
The falls at Niagara have moved approximately 11 kilometres (6.8 mi) upstream from the Niagara Escarpment in the last 12,000 years, resulting in a gorge below the falls. The fine particles created by the erosion has been carried downriver and deposited at the mouth of the river.
However, about 10,500 years ago, through an interplay of geological effects including alternating retreats and re-advances of the ice, and rebounding of the land when released from the intense pressure of the ice (isostatic rebound), this process was interrupted. The glacial melt waters were rerouted through Northern Ontario, bypassing the southern route. For the next 5,000 years, Lake Erie remained only half the size of today, the Niagara River was reduced to about 10 percent of its current flow, and a much-reduced falls stalled in the area of the Niagara Glen.
About 5,500 years ago, the melt waters were once again routed through Southern Ontario, restoring the river and falls to their full power. Then the falls reached the whirlpool.
It was a brief and violent encounter: a geological moment lasting only weeks, maybe even only days. In this moment, the falls of the youthful Niagara River intersected an old riverbed, one that had been buried and sealed during the last Ice Age. The falls turned into this buried gorge, tore out the glacial debris that filled it, and scoured the old river bottom clean. It was probably not a falls at all now but a huge, churning rapids. When it was all over, it left behind a 90-degree turn in the river we know today as the Whirlpool, and North America’s largest series of standing waves we know today as the Whirlpool Rapids.
Why is the water so green?
The startling green colour of the Niagara River is a visible tribute to the erosive power of water. An estimated 60 tons of dissolved minerals are swept over Niagara Falls every minute. The colour comes from the dissolved salts and “rock flour,” very finely ground rock, picked up primarily from the limestone bed but probably also from the shales and sandstones under the limestone cap at the falls.
At the butt of a river, the change in flow condition causes the river to drop any sediment it is carrying. Sediment deposition can generate a variety of landforms, such as deltas, sand bars, spits, and tie channels. Landforms at the river mouth drastically alter the geomorphology and ecosystem.
Is a sandbar formed by erosion or deposition?
Sand bar: A strip of land formed by deposition of sediment via longshore drift or at the mouth of a river.
Barrier Island: A sandbar disconnected from the land. They form due to longshore drift and protect shallow brackish bays or salt marshes behind them. They general form in areas of low shore gradient.
How are sand bars created?
Sandbars begin forming underwater. As waves break, this pulls material from the shoreline, migrating further into the lake. During heavy storms, large waves can build sandbars far from shore, until they rise above the water’s surface.
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Logging requirements:
- Can you easily get to the water at the observation location? Why?
- What type of landform has been created at the mouth of the Niagara River? Sandbar or Barrier Island
- Can you see any 'breakers' or any other disturbance in the water? Would this help a sailor to see the location the bar? Estimate how far offshore?
- What colour is the water of the Niagara River and why is the river this colour?
Photos of the lake at the time of your visit would be appreciated.
