Alabama Landscapes

Rivers and Lakes

Most of Alabama's major rivers are permanent, that is they flow year round , irrespective of rainfall. The reason that happens is that the rivers receive most of their water from groundwater, not from direct precipitation or runoff. The groundwater comes from rain that has soaked into the soil and slowly moved through the underlying rocks and into the river's channel (Figure RL4).

Figure RL3.  Relations between surface and groundwater.

Measuring River Flow

The amount of water that a stream carries is measured by its discharge. Discharge is the volume of water a river carries at a particular point in a set period of time and is measured in cubic feet per second (cfs). Discharge is measured at a gauging station (Figure RL4) and the results are shown on a hydrograph (Figure RL5).

Figure RL4. Gauging station, Cahaba River, Trussville.  (Image © Mike Neilson)

Figure RL5.  Hydrograph for Cahaba River, Trussville (USGS)

Flooding

Flooding occurs when heavy rainfall in a river basin increases stormflow to the point that discharge is greater than the river channel can hold (Figure RL6).

Figure RL6. Shades Creek almost at flood stage after 8" of rain. Birmingham, Jefferson Co. (Image © Mike Neilson)
Based on historic rainfall and flooding records it's possible to estimate the frequency of large floods in a specific area and the heights to which flood waters will rise of floods of different sizes. Generally small floods are more common than large ones. A one-hundred year flood is a flood whose size is such that it has a 1% chance of occurring in a year. The area that will be covered by water in a one-hundred year flood is called a flood plain.

Lakes

Most of Alabama's lakes (actually reservoirs) are human-made, the result of TVA and Alabama Power harnessing the Tennessee and Coosa Rivers for hydroelectricity (Figure RL1).

However, in the southern part of the state along the Florida border are several large natural lakes, including Lake Jackson near Florala. These lakes formed in depressions caused by the solution of the underlying limestones.

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