1. Glacial Erosion:
* Overdeepening: As glaciers flow, their immense weight and abrasive power carve out valleys and basins, often creating depressions much deeper than the surrounding landscape.
* Tributary Glaciers: Smaller glaciers flowing into larger ones often create relatively high rims or divides between the tributary valleys. This creates a natural damming effect.
2. Glacial Meltwater and Water Flow:
* Meltwater Accumulation: During warmer seasons or interglacial periods, glacial meltwater accumulates in the depressions created by the glacier.
* Water Pressure: The water trapped behind the "dam" of the glacial divides builds up pressure.
* Erosion and Overflow: The pressure eventually overcomes the resistance of the glacial ice or moraine material. This triggers a sudden and powerful outflow of water, eroding a channel (the spillway) through the blockage.
3. Spillway Formation:
* Erosive Power: The torrent of meltwater carries massive amounts of sediment, further deepening and widening the channel.
* Downcutting: As the water flows downhill, it erodes the bedrock, creating a steep-sided valley or canyon known as a spillway.
4. Types of Spillways:
* Ice-marginal Spillways: Occur along the edge of a glacier, often forming where a tributary glacier joins the main ice flow.
* Subglacial Spillways: Form beneath the glacier, often associated with tunnels or channels carved through the ice. These can sometimes emerge as dramatic waterfalls at the glacier's terminus.
* Glaciofluvial Spillways: Develop after a glacier has retreated, leaving behind a network of channels and terraces carved by the meltwater.
5. Evidence of Spillways:
* Steep-sided valleys and canyons: Often with a U-shaped profile.
* Terraces and benches: Created by the deposition of sediment along the sides of the spillway.
* Outwash plains: Large, flat areas of sediment deposited by the meltwater.
Examples:
* The Channeled Scablands in Washington State, USA: A classic example of a glaciofluvial spillway system.
* The Grand Canyon of the Yellowstone: Formed by the erosive power of meltwater from the Yellowstone glaciers.
In conclusion, glacial spillways are dramatic features formed by the erosive power of meltwater. They are testament to the tremendous forces involved in glacial processes and provide valuable insights into the history of glaciation.
