9.5: How Is Streamflow Measured?
How Is Streamflow Measured?
The United States Geological Survey (USGS) is one of the major organizations tasked with measuring streamflow here in the US. In general, discharge Links to an external site. measures stream flow at a given time and location. It is a measurement of the volume of water passing a particular point during a period of time. Discharge can be calculated by using the following equation, Discharge = Area * Velocity. Area is determined by multiplying the width by the depth of the stream channel. Velocity of the water is typically measured in units of feet per second or meters per second, while discharge has units of cubic feet per second (cfs) or cubic meters per second (cfm). Discharge increases downstream in most rivers, as tributaries join the main channel and add water. More information about how the USGS measures streamflow is available via their Streamflow Measurement website Links to an external site..
Sediment load also changes from headwaters to mouth. At the headwaters, tributaries quickly carry the load downstream, combining with loads from other tributaries. The sediment is eventually deposited when the stream reaches base level Links to an external site.. Occasionally during this process of transporting material downstream, the sediment load is large enough that the water is not capable of transporting it, so deposition occurs. If a stream becomes overloaded with sediment, a braided stream Links to an external site. may develop. Typically, these streams have a network of intersecting channels that resembles braided hair with sand and gravel bars common. Braided streams are common in sediment-abundant areas, near glaciers or in arid and semiarid regions with high erosion rates.
Figure 9.10: Left: Parts of a meandering stream. The S-curves are meanders. The arrows within the stream depict where the fastest water flows. That water erodes the outside bank, creating a steep bank called the cut bank. The slowest water flows on the inside of the meander, slow enough to deposit sediment and create the point bar. Right: Formation of an oxbow lake. A meander begins to form and is cut off, forming the oxbow. (CC-BY-SA 2.5, Maksim Links to an external site.; modified by Chloe Branciforte)
Streams may also be meandering Links to an external site., with broadly bending meanders that resemble “S”-shaped curves. Water will travel the fastest on the outside of a bend. This higher velocity leads to more erosion on the outside bend, forming a cut bank Links to an external site.. Erosion at the cut bank is offset by deposition on the opposite bank of the stream, where the water moves slower and allows sediment to settle out and deposit. These areas of deposition are called point bars Links to an external site.. As meander bends become more complicated, or sinuous, they may intersect to form a cutoff, which shortens the stream’s path. After cutoff the abandoned meander loop becomes a crescent-shaped oxbow lake Links to an external site. (Figure 9.10 and 9.11).
Figure 9.11: Klamath River, illustrating the cut bank and point bar areas. (CC-BY 4.0; Chloe Branciforte via Google Earth)
Straight streams, in which channels remain nearly straight, are uncommon. They can form naturally due to a linear zone of weakness in the underlying rock or can be human constructed, in an effort at flood control, much like what has been done with the Los Angeles River Links to an external site. (Figure 9.12).
Figure 9.12: Aerial photo of the Los Angeles River. (CC-BY-SA 3.0; Joe Mabel Links to an external site.)