A Tale of Two Rivers
In Geneva, Switzerland, two rivers meet in a display of geologic force. The clear, blue-green waters of the Rhône, having shed their sediment in the depths of Lake Geneva, flow alongside the pale, milky torrent of the Arve River. For a considerable distance, they refuse to mix, a visual boundary created by differences in water density, temperature, and speed. The Arve's distinctive color is not from pollution, but from its immense load of "glacial flour."
This flour is rock ground to a fine powder by the immense weight and slow movement of glaciers in the Mont Blanc massif, the Arve's source. As glaciers like the famous Mer de Glace advance, they scrape and pulverize the bedrock beneath them, creating silt and clay-sized particles, typically just a few micrometers in diameter. This material, composed mainly of ground-up quartz and feldspar, is so fine that it remains suspended in the meltwater, scattering light and giving the river its opaque, milk-like appearance.
Quantifying an Eroding Landmark
The Arve River erodes the Alps, carrying a massive amount of the Alps downstream. Estimates suggest the river transports between 1 and 3 million cubic meters of sediment each year. Near Geneva, about 400,000 cubic meters of this sediment settles annually in the reservoir of the Verbois dam. The concentration of suspended sediment in meltwater rivers like the Arve can commonly range from 0.1 to over 1 gram per liter.
This process measures the mountains being worn away. While tectonic forces push Mont Blanc upwards by about 1 millimeter per year, glacial and sub-aerial erosion work to wear it down. Studies on the flanks of the Mer de Glace show post-glacial bedrock erosion rates ranging from 3.5 to 4.3 millimeters per year, indicating the powerful effect of these grinding and weathering processes.
The river's flow and sediment load fluctuate dramatically with the seasons. The discharge is highest during the summer melt, from June to August, when the flow of glacial milk is at its peak. The Arve's mean annual discharge at its mouth in Geneva is approximately 79 cubic meters per second. This constant scouring and deposition of sediment has a significant impact on the river's ecosystem, increasing turbidity which limits light for aquatic plants and can affect fish populations.