Page 290 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

254 part II The Water, Weather, and Climate Systems
Snow and Ice
The largest amount of surface freshwater on Earth is stored in glaciers, permafrost, and polar ice (review Figure 9.3). Seasonal melting from glaciers and the annual snowpack in temperate regions feeds streamflow, contributing to water supplies. Snowpack melting captured in reservoirs behind dams is a primary water source for humans in many parts of the world.
Glaciers provide a semipermanent form of water storage, although rising temperatures associated with re- cent climate change are causing accelerated rates of gla- cial melting. The residence time of water in glaciers can range between decades and centuries, and the relatively small but continuous meltwater from glaciers can sustain streamflow throughout the year (Chapter 17 discusses glaciers and ice sheets).
Rising temperatures have caused glacier melt rates to accelerate, and some scientists estimate that most glaciers will be gone by 2035 if present melt rates con- tinue. On Asia’s Tibetan Plateau, the world’s largest and highest plateau at 3350 km elevation, climate change is causing mountain glaciers to recede at rates faster than anywhere else in the world. More than 1000 lakes store water on this plateau, forming the headwaters for sev- eral of the world’s longest rivers. Almost half the world’s population lives within the watersheds of these rivers; the Yangtze and Yellow Rivers (both flowing westward through China) alone supply water to approximately 520 million people in China. Even as glacial melting has increased streamflows, the worsening drought in west- ern China is causing these flows to evaporate or infiltrate the ground before reaching the largest population centres. Although disappearing glaciers will not signifi- cantly change water availability in the lower-elevation regions, which depend on monsoonal precipitation and snowmelt, these changes will affect high-elevation water supplies, especially during the dry season.
Rivers and Lakes
Surface runoff and base flow from groundwater move across Earth’s surface in rivers and streams, forming vast arterial networks that drain the continents (Figure 9.13). Freshwater lakes are fed by precipitation, streamflow, and groundwater, and store about 125000 km3, or about 0.33%, of the freshwater on Earth’s surface. About 80% of this volume is in just 40 of the largest lakes, and about 50% is contained in just 7 lakes (Figure 9.13).
The greatest single volume of lake water resides in 25-million-year-old Lake Baikal in Siberian Russia. This lake contains almost as much water as all five North American Great Lakes combined. Africa’s Lake Tanganyika contains the next largest volume, followed by the five Great Lakes. About one-fourth of global freshwater lake storage is in small lakes. More than 3 million lakes exist in Alaska alone; Canada has at least that many in number and has more total surface area of lakes than any country in the world.
Not connected to the ocean are saline lakes and salty inland seas, containing about 104000 km3 of water. They usually exist in regions of interior river drainage (no outlet to the ocean), which allows salts resulting from evaporation over time to become concentrated. Examples of such lakes include Utah’s Great Salt Lake, California’s Mono Lake and Salton Sea, Southwest Asia’s Caspian and Aral Seas, and the Dead Sea between Israel and Jordan.
Effects of Climate Change on Lakes Increasing air temperatures are affecting lakes throughout the world. Some lake levels are rising in response to the melting of glacial ice; others fall as a result of drought and high evaporation rates. Longer, warmer summers change the thermal structure of a lake, blocking the normal mix- ing between deep and surface waters. Lake Tahoe in the Sierra Nevada mountain range along the California– Nevada border is warming at about 1.3 C° per decade. The rate of warming is highest in the upper 10 m, and mixing has slowed. Non-native, invasive species such as large-mouth bass, carp, and Asian clam are on the rise in warming lakes, while cold-water species decline.
In East Africa, Lake Tanganyika is surrounded by an estimated 10 million people, with most depending on its fish stocks, especially freshwater sardines, for food. Present water temperatures have risen to 26°C, the highest in a 1500-year climate record exposed by lake- sediment cores. The mixing of surface and deep waters is necessary to replenish nutrients in the upper 200 m of the lake, where the sardines reside. As mixing slows or stops, scientists fear that fish stocks will continue to decline.
Hydroelectric Power Human-made lakes are generally called reservoirs, although the term lake often appears in their name. Dams built on rivers cause surface water res- ervoirs to form upstream; the total volume worldwide of such reservoirs is estimated at 5000 km3. The largest res- ervoir in the world by volume is Lake Kariba in Africa, impounded by the Kariba Dam on the Zambezi River on the border between Zambia and Zimbabwe. The third and fourth largest are also in Africa (Figure 9.13).
Although flood control and water-supply storage are two primary purposes for dam construction, an associ- ated benefit is power production. Hydroelectric power, or hydropower, is electricity generated using the power of moving water. Currently, hydropower supplies almost one-fifth of the world’s electricity and is the most widely used source of renewable energy. However, because it depends on precipitation, hydropower is highly variable from month to month and year to year.
China is the leading world hydropower producer. The Three Gorges Dam on the Yangtze River in China is 2.3 km long and 185 m high, making it the largest dam in the world in overall size, including all related con- struction at the dam site (Figure 9.14). Entire cities were relocated (including more than 1.2 million people) to