Figure 1
.
Sediment and Solute Fluxes in Rivers:
Rating curves quantify the relationship between sediment flux and discharge (Figure 1). Sediment flux can be divided into suspended load and bedload transport. Suspended load is distributed throughout the channel flow while bedload is the soil, rock particles, or other debris carried along the river bed. In most rivers excepting small mountainous streams suspended load exceeds 90% of the total load.
Figure 1
The global distribution of sediment yield (sediment flux
per unit drainage area) is shown in Figure 2.
The dominant control on
suspended sediment
load is drainage area. Four rivers that have especially
high (the Haiho or Yellow River and the Brahmaputra) and low (Rhein and
St. Lawrence Rivers) sediment yields. The Yellow River has a high sediment
yield primarily because it drains basins of easily erodible loess. The
Brahmaputra has a high sediment concentration because it drains some of
the steepest topography in the world along the Himalayan front.
Conversely,
the Rhein or Rhone in Germany flows through very gentle topography of
highly
resistant bedrock for much of its course. The St. Lawrence dumps much of
its sediment in the Great Lakes before it reaches the Atlantic. The
highest sediment yields in the world are located in New Zealand and Taiwan
which also have the highest uplift rates. This strongly suggests
that in very active orogens there is an approximate equilibrium between
uplift and denudation. The main contributing factors in determining
sediment yield are
The dramatic effect of land use is shown in Figure 3. The effect of grazing lands can increase the sediment yield by orders of magnitude over agricultural and forested land. The effects of land use are greater for greater runoff.
Figure 3
The effects of dams should always be considered when working with sediment load data because they can have a huge impact. The water and sediment discharge for the Colorado River at Yuma, AZ downstream of Hoover Dam is shown in Figure 4. The effect of the dam is to lower the average water discharge in Yuma by a factor of 2 to 3. The sediment discharge is lowered by an even greater amount because the highest sediment concentrations are close to the bed. The water that does make it past the dam is thus depleted in sediment and that results in a lower sediment concentration downstream of a dam compared to upstream.
Figure 4
.
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