WATERSHEDS

A watershed is an area of land that drains water, sediment, and dissolved materials to a common outlet at some point along a stream or channel (Dunne & Leopold, 1978).  Watersheds occur at multiple scales from a large river basin such as the Mississippi River to a small headwater stream.  The defining features of a watershed include drainage divides, hillslopes, floodplains, terraces and deltas.  The boundary of a watershed is based on the drainage divides, which take into account both hydrologic and geomorphic processes.  Other determining factors include climatic regime, topography, underlying geology, morphology, soils, and vegetation. (USDA, Stream Corridor Restoration, 1999)

Watershed assessment is a necessary component of a monitoring program in order to determine what degraded or impaired areas may exist in the watershed and why. Several characteristics of a watershed are taken into consideration during the assessment process including land use, land cover, and hydrology. Land use and cover is considered in a historical as well as current perspective to determine the types of activities that have occurred in the watershed and their potential as sources of pollution. It is important to consider the natural and cultural resources of the watershed as well as the human activities. This information may be obtained from various sources including topographic maps to determine drainage area and land features as well as land use data.

LAND USE

Various land use practices contribute pollutants to lakes, streams, and estuaries.  Closely examining which land use practices occur in your watershed may help in the determination of pollutants of concern.  Construction sites disturb the natural land cover during the excavating and grading processes.  Soil erosion is increased due to the exposed land and removal of protective vegetation. Contaminants such as oil, grease, paints, glues, and acids may be washed off site by stormwater runoff over the newly exposed landscape, into the nearby streams or lakes.  Also, failure to design and construct projects with water quality in mind leads to increased runoff over impervious surfaces to the nearby waterbody thereby increasing the peak flow intensities which causes further damage to streambanks and habitat. The hydrograph below depicts the discharge during a rain event for pre and post development conditions. Another concern associated with development in the watershed is the misuse of septic systems.  Improper design, installation, or operation of septic systems or holding tanks may lead to pollution of ground and surface waters by bacteria, nutrients, and household toxic chemicals.  Urbanization within the watershed not only concentrates people, but also concentrates the pollutants resulting from their lifestyles.  Urban areas are characterized by largely impervious surfaces such as buildings, streets, sidewalks, and parking lots.   These surfaces do not allow for infiltration of rainwater and as a result, the stormwater runoff leads to the nearby streams and lakes via drains and culverts.  The type and amount of pollutants that are carried in this runoff are influenced by several factors including traffic density, littering, fertilizer and pesticide use, construction site practices, animal wastes, soil characteristics, topography, atmospheric deposition and amount of precipitation.  The potential urban pollutants include nutrients, bacteria, sediment, toxic chemicals and litter.  Aside from urban concerns, there are other land use practices related to natural resources which influence water quality.  Mining activities may pollute lakes, streams, and groundwater with sediment, dust, chemicals, and wastes from open pit, strip, and underground mines.  Also, forestry practices, which are usually associated with high water quality, may have negative impacts during certain periods of site preparation and harvesting.  Road construction, clearing of land for fire breaks, stacking and loading operations during harvest, mechanical site preparation, and application of pesticides and herbicides may all contribute pollutants to the nearby water resource.  Agriculture, including livestock operations as well as cropland, impacts water quality in a variety of ways.  With livestock operations, feed lot management and waste management are major concerns.  Treatment of the runoff from feedlots is essential to reduce the levels of nutrients and bacteria entering nearby water bodies.   Animal waste must be managed to reduce, as much as possible, the amount of nutrients leaving the site.  Cropland agriculture may contribute sediment, nutrients, bacteria, and organic contaminants due to excess fertilizer and pesticide applications, poor erosion control, and excess runoff.   The most significant of these pollutants associated with cropland is sediment.  While each of these land use practices have the potential to degrade water quality, with proper best management practices and awareness of the water resources, the damage may be minimized. Table 3.3 shows a detailed representation of the potential effects of major land use activities.

HYDROLOGY

The hydrologic cycle (figure ?) is a continuum of the transfer of water from precipitation to surface water and ground water to storage and runoff, and to the eventual return to the atmosphere by transpiration and evaporation. Evaporation occurs when water on the Earth’s surface is converted to water vapor by the sun’s energy. Transpiration is a similar process and joins with evaporation to be collectively referred to as evapotranspiration. Water returns to the Earth’s surface from the atmosphere in the forms of precipitation and condensation when the water vapor that has entered the atmosphere cools enough to fall to the Earth. When precipitation occurs, the water reaching the Earth’s surface may become part of the stream flow in a variety of ways. The precipitation that falls directly into a stream is called channel interception, the water reaching the stream from the surface is runoff or overland flow. Some of the precipitation may infiltrate into the soil and move to the water table, which moves laterally and down-gradient towards the stream contributing base flow as well as lateral flow. The base flow is the ground water contribution to the stream that sustains its flow during periods of little precipitation.

A hydrograph plots the discharge of a stream in cubic feet per second (cfs) over time. Discharge is the volume of water moving through a channel per unit time (Q=A*V; A = area through which the water is flowing, V = velocity in feet per second at a specific location). During precipitation events, the discharge of a stream increases as more water enters the stream channel. Likewise, a hydrograph will show a storm event as a noticeable change in discharge over time, which will then return to normal flow rates once the storm, has past. In areas of little development such as forests, a hydrograph will rise slowly during the storm and gradually decrease as well. This is due to the increased infiltration of precipitation into the soil, less runoff or overland flow, and increased interception by vegetation. In more urban areas where the percentages of impervious surfaces are higher, storm hydrographs will tend to peak quickly and drop quickly as well. This is a result of increased surface runoff, as many of the previous areas of infiltration are now impervious and the water is directed to the streams by a series of culverts and drainage pipes.

Figure 1.1 Hydrograph before and after urbanization in the watershed

DIFFERENCES IN HYDROLOGY BETWEEN WATERBODIES

The differences between streams, and lakes and wetlands, hydrologically have direct influences on the water quality and the ability of the water body to handle pollutants. Lakes and non-tidal wetlands have relatively little movement of water throughout, which may increase the retention time for pollutants. The exposure of the aquatic and terrestrial organisms to the pollutants is also increased. In a stream environment however, the water carries pollutants away, which may contaminate areas downstream, yet offers more dilution. Lakes, ponds, and wetlands are very similar systems with the main difference being in water depth and vegetation. Lakes typically are deeper than ponds, and wetlands may be classified in a variety of categories including swamps, bogs and marshes. Some lakes and ponds may eventually become wetlands if they receive high amounts of sediment and become shallower. This however, does not always occur. Areas with increased development may create this phenomenon by dramatically increasing the sediment load reaching the lakes and ponds due to high erosion rates from areas of exposed soils. Wetlands do serve important purposes hydrologically by controlling floodwaters, containing sediment, filtering contaminants, recharging aquifers, and providing habitat.

Streams and rivers are characterized by moving water, which results in decreased temperatures and increased dissolved oxygen levels. Streams may be described from their point of origin, in the headwaters, to the rivers that empty into the ocean. The order in which streams flow from headwaters to the river follows a hierarchy and the streams are classified according to their number of tributaries (Strahler, 1957). A stream in the headwaters which has no tributaries emptying into it, is considered a first order stream. Further down the hierarchy, second, third, and fourth order streams become part of the network. In the upper reaches, or headwaters, there are three different types of streams that are termed according to their timing and amount of streamflow. Ephemeral streams flow only during or immediately after periods of precipitation.  They generally flow less than 30 days per year.  Intermittent streams flow only during certain times of the year; the seasonal flow usually lasts for longer than 30 days a year.  Perennial streams flow continuously during both wet and dry seasons during both wet and dry times; it is dependably generated from the movement of groundwater into the channel. Streams may also have a variety of drainage patterns depending on the topography, geology, vegetation, and other factors within the watershed. Drainage patterns such as dendric, radial, and contorted are some examples.                           

CONCLUSION

When determining the hydrology and activities within the watershed of concern, be sure to search for existing data to avoid duplicating efforts. In North Carolina, the Division of Water Quality and the Division of Water Resources have collected information on the seventeen river basins, and continue to publish basinwide reports. The US EPA provides detailed information for hydrologic units across the entire state in the BASINS database, as well as information about NC  watersheds on the Surf Your Watershed web page (http://www.epa.gov/surf2/states/nc.html). The USGS may provide information about your watershed through their gaging stations (http://waterdata.usgs.gov). Also, your local government has  information about zoning, industry, commercial operations, discharge permits, and water usage. All of this may be helpful in your preliminary watershed assessment.

Locating the potential sources of impairment and pollution within the watershed aids tremendously when deciding when and where to collect your water quality samples. For example, you may choose to sample up and down stream of a known source of pollution to determine the extent of its contamination. Sources of pollution are not a requirement for a monitoring program, however, many programs monitor their water quality to determine damage by non-point sources or to determine that their water resources are of high quality.