Changing sedimentation levels in rivers can have both positive and negative impacts on human activities. Positively, alluvium deposited from rivers enriches soils for farmers along riverbanks. However, sediment deposition also limits channel capacity and increases flooding risks. It blocks irrigation channels, fills lakes and reservoirs reducing water storage, and requires frequent dredging of shipping channels. Large amounts of sediment are deposited daily in rivers like the Colorado River and Lake Powell, and the Yellow River carries over 1.6 billion tons of silt annually, depositing much of it in its bed and changing its course. Dams and other infrastructure aim to control flooding while capturing water for irrigation and hydropower, but the high sediment loads threaten their long

1. Changing Sedimentation Levels -impact of rivers on human activities

3. Positive or negative? Positive Alluvium deposited from rivers is rich in nutrients & can benefit riverside farmers Negative Limits channel capacity & increases chance of flooding Blocks irrigation channels Infills lakes & reservoirs –reduces storage Blocks shipping channels –need frequent dredging Reduces sunlight making conditions unsuitable for some aquatic life

4. Factors that affect sediment input Vegetation Climate Soils Rock types Slope gradients Drainage basins Downslope ploughing Dams urbanisation

5. Colorado River The equivalent of 30,000 dump truck loads of sediment is deposited into Lake Powell every day!

6. The Huang He- Yellow River The Yellow River is notable for the large amount of silt it carries—1.6 billion tons annually at the point where it descends from the Loess Plateau . If it is running to the sea with sufficient volume, 1.4 billion tons are carried to the sea. In modern times, since 1972 when it first dried up, the river has dried up in its lower reaches many times, from Jinan to the sea in most years, in 1997 for 226 days. The low volume is due to increased demand for irrigation , by a factor of five since 1950. Water diverted from the river as of 1999 served 140 million people and irrigated 74,000 km² (48,572 mi²) of land. The highest volume occurs during the rainy season, from July to October, when 60% of the annual volume of the river flows. Unfortunately, water for irrigation is needed between March and June. In order to capture excess water for use when needed, and for flood control and electricity generation, several dams have been built, but due to the high silt load their life is expected to be limited. A proposed South-North Water Transfer Project involves several schemes to divert water from the Yangtze River , one in the western headwaters of the rivers where they are closest to one another, another from the upper reaches of the Han River , and a third using the route of the Grand Canal . Due to its heavy load of silt the Yellow River is a depositing stream , that is, it deposits part of its carried burden of soil in its bed in stretches where it is flowing slowly. These deposits elevate the riverbed which flows between natural levees in its lower reaches. Should a flood occur, the river may break out of the levees into the surrounding lower flood plain and adopt a new course. Historically this has occurred about once every hundred years. In modern times, considerable effort has been made to strengthen levees and control floods. The Yellow River delta totals 8,000 square kilometers (3,090 mi²). However, since 1996 it has been reported to be shrinking slightly each year through erosion ( From Wikepedia)