8 Running Water.docx - 8 Running Water Outline 8.1 8.2 8.3...

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8. Running Water Outline 8.1 Hydrologic Cycle 8.2 Stream Flow 8.3 Stream Erosion 8.4 Stream Deposition 8.5 Stream Maturity 8.6 Flooding 8.1 Hydrological Cycle Of the total volume of water precipitated on Earth’s surface each year, about 90% returns to the atmosphere through evapouration and transpiration, 10% runs off in streams to the oceans, and less than 1% infiltrates into the ground to become groundwater (Figure 8.1). Groundwater (covered in Chapter 9) migrates through bedrock and regolith, with most of it ultimately working its way back to the surface as springs on land or beneath the oceans. In cold climates, a significant proportion of precipitation freezes temporarily, becoming part of a snowfield or glacier (covered in Chapter 10). This transfer of water from the atmosphere to the land and oceans and back again is called the hydrologic cycle . A great deal of water is also chemically bound to minerals in the crust and mantle. This water too is part of the hydrologic cycle as it enters the atmosphere from hot springs and volcanic vents and returns to rock through weathering, crystallization and subduction. However, we will ignore water bound in minerals here since it was covered in the chapters on mineralogy and igneous, sedimentary and metamorphic rocks. Since the volume of water in the oceans is not changing very much, the hydrologic cycle is more or less in balance. This may have been the case for a long time since some of the oldest known rocks, dated at just less than 4 Ga, are metamorphosed conglomerates which must have been deposited in streams or beaches. The hydrologic cycle is powered primarily by the energy of the sun. The 380,000 km 3 of water that cycles annually through the atmosphere is sufficient to cover Earth’s surface to a uniform depth of about 1 metre. Of the five main repositories of free water on Earth precipitation evapouration 90% infiltration <1% runoff 10% Figure 8.1. Cartoon illustrating the hydrologic cycle - of the volume that precipitates each year, 90% is evapourated or transpirated, 10% runs off to the oceans, and <1% infiltrates. transpiration + 1
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(atmosphere, fresh water streams and lakes, groundwater, glaciers, and oceans), fresh water streams and lakes are the most critical to humans since we rely on them for much of our water supply. Unfortunately, stories of insufficient or contaminated water supplies are becoming increasingly frequent in the news. This should not be surprising, given the small proportion of water supply that is readily available for community use – if we imagine that all free water on Earth is equivalent to a 5 L bucket, only 25 ml would be fresh water and only 0.3 ml would be fresh water in surface streams and lakes (see Figure 8.2). It is this running water that we are concerned about in this chapter.
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