Channel dynamics over long timeframes

The prior discussion describes how changes in geomorphic processes along the river's length help us understand the development of different channel configurations and features. It emphasizes how adjustments are continually occurring to maintain an approximate equilibrium, and emphasizes events of intermediate magnitude -the dominant discharge - as primarily responsible. In a historical perspective, we should also emphasize the importance of episodes of climate change over the last 15,000 years, and of even older tectonic events and glaciations. For example, the Grand Canyon of the Colorado River is thought to be about 5 million years old and its establishment to depend ultimately on tectonic movements and the opening of the Gulf of California. Both ancient and more recent floods have on occasion had lasting effect on fluvial landscapes, carving channels and placing large clasts that subsequent floods are unable to substantially modify (Knighton 1998). River channels in a large area of the western United States were shaped by a paleoflood estimated to have discharges as high as 10 million m3 s 1 (Benito 1997), which occurred when an ice dam on glacial Lake Missoula failed some 15,000 years ago. Other ice age floods also had grand effects. Less spectacularly, climatic fluctuations since 10,000 years ago have affected water balances, vegetation patterns, flows, and the supply of materials, resulting in fluctuations in fluvial activity. The postglacial history of a 250 km section of the Mississippi River from the confluence of the Missouri and Ohio Rivers to south of

Memphis, Tennessee, illustrates how episodes of major flood influence have resulted in a series of abandoned channels and sedimentary deposits (Blum et al. 2000). More recently, the Mississippi River experienced large floods between AD 1300 and AD 1500 during the period of transition into the Little Ice Age (Knox 1993).

These climatic influences continue into the present and will be important if future climate change takes place as forecast. However, it has become increasingly difficult to isolate climate from the influence of human activity, particularly land clearing. Deforestation in parts of the Huang Ho (Yellow River) catchment in China between 200 BC and AD 600 is estimated to have increased sediment loads by an order of magnitude (Milliman et al. 1987). During the gold rush of the mid-19th century in the Sierra Mountains of California, hydraulic mining mobilized great amounts of waste gravel into the headwaters of the Sacramento River (James 1991). The lower Bear River aggraded as much as 5 m, and is continuing to adjust over 100 years later, which points to the very long time frame of river recovery following human disturbance. In some instances recovery may not be possible, as when river impoundment eliminates flood peaks, or when invading vegetation stabilizes the new configuration, as has occurred in some lower sections of the Bega catchment of Australia (Brierley et al. 1999).

0 0

Post a comment