4 Rivers and Streamflow

Measuring river flow

Rivers are moving-water systems that connect hillslopes, groundwater, lakes, wetlands, and downstream basins. Their flow shapes channels, carries sediment, and periodically spreads onto floodplains. River form changes over time as water and sediment move through the system.

Streamflow, also called discharge and represented by $Q$, is the volume of water passing a point per unit time. It is commonly reported in cubic feet per second, $\mathrm{ft^3/s}$, or cubic metres per second, $\mathrm{m^3/s}$. Discharge is related to the channel's wetted cross-sectional area and average water velocity:

Here, $A$ is the wetted cross-sectional area and $\bar{v}$ is the average water velocity. For example, a wetted cross-sectional area of $12\,\mathrm{m^2}$ and an average water velocity of $2\,\mathrm{m/s}$ give a discharge of $24\,\mathrm{m^3/s}$.

Stage is the water-surface height at a particular location, so it is not the same as discharge. A streamgage can use a locally established stage–discharge relationship to estimate flow. Flow varies with precipitation, snowmelt, groundwater inflow, evaporation, and water use or storage. A hydrograph shows how stage or discharge changes over time. Small basins may rise and fall quickly after a storm, while larger basins often respond more slowly because water takes longer to travel through them.

Drainage networks and channel form

A drainage basin, also called a watershed, is the area that drains water to a common outlet. A drainage divide, often following ridges or other high ground, separates neighboring basins. Small streams join to form larger streams and rivers. A tributary is a smaller stream entering a larger one, and their meeting point is a confluence.

Water and sediment move through this connected network toward an outlet, although some water may infiltrate, evaporate, or be stored along the way. The network connects the land and water systems that contribute flow to downstream basins.

A channel's width, depth, slope, bed material, vegetation, and pattern reflect interactions among water flow, sediment supply, and valley conditions. Some rivers occupy relatively straight channels; others meander in one winding channel or split into multiple, shifting channels. These forms are not fixed: floods, changes in sediment supply, vegetation, and human structures can alter them.

Erosion, deposition, and sediment

In a meandering river, faster flow commonly erodes the outer bank, forming a cut bank, while slower flow deposits sediment on the inner bank, forming a point bar. Continued erosion and deposition can shift bends sideways and downstream. If a bend is cut off from the main channel, it may leave an oxbow lake.

Flowing water erodes the bed and banks by loosening and removing material. Its ability to erode and carry sediment depends on factors such as discharge, velocity, channel slope, bed and bank material, and vegetation. When flow slows or its carrying capacity falls, sediment may settle and accumulate. When flow gains energy, stored sediment may be picked up and moved again.

Rivers transport sediment in several ways:

• Dissolved load: minerals carried in solution.

• Suspended load: fine particles, such as silt and clay, carried within the water.

• Bed load: coarser particles, such as sand or gravel, moved along the bed by rolling, sliding, or hopping.

Erosion and deposition occur in different places and at different times. For example, a flood may erode a bank, move gravel along the channel bed, and deposit fine sediment across a floodplain as water spreads out and slows.

Floods and floodplains

A flood occurs when high streamflow overtops a river's natural or artificial banks. The adjacent low, relatively flat land that may be inundated is the floodplain. Floodwaters can deposit sediment there, helping build floodplain surfaces and features such as natural levees.

Flooding can result from intense or prolonged rain, rapid snowmelt, or other conditions that bring more water into a channel than it can contain. Flood magnitude and timing depend on the storm, basin size and shape, slope, soil and vegetation, and human changes such as dams and urban development.

A flood's recurrence interval describes its probability in a given year, not a regular schedule. A 100-year flood has a $1\%$ chance of being equaled or exceeded in any one year.

Floods can reshape channels and transport large quantities of sediment. Floodplains provide space for water to spread beyond the channel. Levees and other barriers may reduce inundation in some places, but they can also limit the river's connection to its floodplain.