Dam and river ecosystem basics module: introduction | hydrology | wetlands | morphology and sediment | water quality | biology | timing of dam repair or removal | references | appendix

Hydrology

Hydrology is the study of water on the Earth’s surface and just under the surface. The types of things hydrologists study can be seen in the water (hydrologic) cycle. Water is constantly cycling between different parts of the Earth and its atmosphere. Imagine a drop of water being evaporated from the ocean into the air, becoming part of a cloud carried by the wind over the land, falling back to Earth as rain, soaking into the soil and flowing with the groundwater to a spring, then flowing over the land surface as part of a river, and back to the ocean (fig. 1).
 

Figure 1. The water cycle. (Figure courtesy of Wisconsin Geological and Natural History Survey.)

The saturated zone is the part of the subsurface in which all the spaces between rock and soil grains are filled with groundwater. The unsaturated zone lies above the saturated zone; there, the spaces between rock and soil grains are filled partly with water, and the remainder is filled with air. The water table is the boundary between the saturated and unsaturated zones.

A river is a natural stream of water of fairly large size that flows over the land surface in a distinct course or channel. How do rivers start? Water flows because of the pull of gravity, and thus rivers start in areas of high elevation, and flow downhill toward lower and lower elevations until eventually reaching the lowest elevation possible, the ocean. Rivers occupy the lowest areas of a landscape, such as the low parts of valleys.

The area of land that captures precipitation and directs it into a river or lake is the watershed of that river or lake. The land of a watershed is at a higher elevation than the water body it drains into, so water flows downhill over this land toward the water body (fig. 2). Because small streams drain into larger rivers, most watersheds are part of a larger watershed. Watersheds for different lakes and rivers are separated from one another by drainage divides, which are ridges of higher elevation. A famous example of a drainage divide is the Continental Divide, which separates land draining to rivers that feed into the Pacific Ocean from land draining to rivers that flow to the Gulf of Mexico. Most drainage divides are not as dramatic, but separate different watersheds nonetheless.
 
 

Figure 2. Diagram of a watershed and its components.
 

Impounded river systems

A dam raises the level of the river water to the height of the dam outlet. This new water level in many cases is above the level of the natural water table in areas adjacent to the impoundment, and some of the water in the impoundment behind the dam flows into the adjacent aquifer, instead of into the river (fig. 3). Mounded groundwater near large impoundments may cause a rise in the water levels of wells near the shoreline.
 

Figure 3. An artificial impoundment creates a groundwater mound near the impoundment.
 
 

If the amount of water released by the dam is equal to the amount of water coming into the impoundment, the water level of the impoundment remains the same. If the amount of water flowing into the impoundment is less than the amount released to the river below the dam, then the level of water in the impoundment falls. If the amount flowing in to the impoundment is more than is released, the impoundment level rises. Most dams that are built for water supply or electricity production vary the amount of water released to the river below the dam. However, most small dams in Wisconsin are “run of the river” dams, meaning that they release the same amount of water that is coming into the impoundment. Thus, these impoundments only store water, rather than regulate the flow of water. They are much like an overflowing bathtub in that the amount of water that spills over the edge of the tub (out from the dam) is equal to the amount that comes in from the faucet (the river upstream).
 

Free-flowing systems

Free-flowing rivers carry water unimpeded by human-built structures. Most of the rivers in Wisconsin receive their water from some combination of natural lakes, rainfall, overland flow, and groundwater. Many rivers in Wisconsin begin as springs, where groundwater rises to the land surface. Rivers also receive groundwater flow directly into the bed of the river. Water in the river originating from groundwater is called baseflow (fig. 4). In many parts of Wisconsin the water table is above the level of rivers and lakes. Because groundwater flows from high to low just as surface water does, groundwater flows into most rivers.
 

Figure 4. The groundwater that flows into a stream is called the stream’s baseflow.
 
 

Discharge is the amount of water a river is carrying. Hydrologists measure discharge in units of volume per time, usually in cubic feet per second. The discharge of a river is the total amount of water from overland flow (including tributary rivers) and the amount of water received from groundwater. The amount of baseflow to a river is relatively constant over time, but is dependent on the regional water-table elevation, which can vary by several feet over one year. However, the discharge of a river varies over time, as the amount of overland flow changes. For example, after a rainstorm or during snowmelt, the amount of water running over the land to the river increases, and discharge of the river then increases. As the discharge of the river increases, does the depth, or stage, of the river increase? The discharge of a river depends on characteristics of the watershed, such as the number of acres, the amount of precipitation received, vegetation, soils, and slope.

Rivers increase in discharge and size with increasing distance downstream. A river in its headwater area (the area where a river begins) is only draining a small part of land. The same river farther downstream is draining a larger area and has received water from tributary streams (smaller streams that flow into a river). The amount of overland flow reaching a river not only depends on the watershed area and amount of precipitation received, but also on the amount of water that soaks into the soil rather than running over the land surface. The water that soaks into the soil is known as infiltration. Infiltration is an important part of the hydrologic cycle because it recharges aquifers, the underground rocks and sediment that contain groundwater. Factors that affect infiltration in a watershed include vegetation and soil types.
 

Hydrologic restoration

When repairing a dam, the water level in the impoundment must be temporarily lowered to allow for construction work. This lowering of the water level (generally referred to as a drawdown of the impoundment) requires allowing a greater outflow of water from the dam outlet than the inflow of water from the river upstream until the most of the impoundment has been emptied. This greater discharge of water cannot be released all at once, causing a flood. Instead, the drawdown must be a slow and controlled process, taking days to weeks to empty the impoundment. During the drawdown, water will cease to flow out of the impoundment into the adjacent aquifer, lowering the groundwater mound. Once the dam has been repaired, the impoundment can be refilled, which is a slow, controlled process. The dam cannot capture all of the river’s flow to fill the impoundment; if it did, the river downstream of the dam would go dry with dire consequences for fish and other wildlife. After the impoundment is refilled, water will again flow out of the bed of the impoundment into the aquifer, re-establishing the groundwater mound near the impoundment.

If a dam is removed, the water level at the land surface drops back to its original level as a free-flowing river, and the water table will return to its original configuration. Problems may arise if shallow wells that depend on the higher water table have been located near the former impoundment. After the removal of the dam, if the water table drops below the bottom of a well, the well will go dry and will need to be replaced with a deeper well. Previous drawdowns of the impoundment for dam maintenance may indicate the extent of impacts on groundwater. However, this is only true if the length of time of drawdown was sufficient to allow the water table to fully lower and that nothing has happened in the watershed that has significantly changed the hydrology of the watershed. Further, if the drawdown occurred during a wet period, the drawdown would not have shown how far the water table would drop during a drought.

As people change the way land is used, they can have a large impact on infiltration in a watershed. Tilling soil and planting different types of vegetation are likely to change infiltration rates. The greatest human impact on infiltration is urbanization. Urbanization increases the area of impervious surfaces, through which water cannot infiltrate. Impervious surfaces include buildings and homes, roads, and parking lots. Because water cannot infiltrate these surfaces, it must run off them. These impervious surfaces reduce groundwater recharge and speed the flow of runoff to rivers, often exacerbating flooding problems (Leopold, 1968). Impervious surfaces can also significantly increase the temperature of a river, thus changing the habitat for cold-water species.
 

Next section: Wetlands

URL: http://www.ies.wisc.edu/research/wrm00/educhydro.htm