Facts About Watershed

Watershed Facts

All land on earth is a watershed. Humans and their activities play important and essential roles within them, yet few people understand watersheds. Still fewer know the dynamics and boundaries of the ones in which they live.

A watershed is the land area from which both surface water and groundwater, sediment and dissolved materials drain to a common watercourse or body of water. For each watershed, there is a drainage system that conveys rainfall to its outlet. A watershed may be the drainage area surrounding a lake that has no surface outlet, or a river basin as large as that of the St. Johns or Suwannee rivers or the Colorado River. Within a large watershed are many smaller watersheds that contribute to overall streamflow.

The point at which the boundaries of two watersheds come together or connect is called a divide. Some areas have small changes in land surface or topographic relief, and the divide between watersheds is subtle. A watershed is drained by a network of channels that increase in size as the amount of water and sediment they must carry increases or by overland sheet flow, which is harder to visualize.

Streams are dynamic, open-water systems that collect and convey surface runoff generated by rainfall, snowmelt, or groundwater discharge to estuaries and oceans. The shape and pattern of a stream are a result of the land it cuts and the sediments it carries, as well as the results of human alterations.

Stream Orders

In most cases, a watershed system is almost entirely made of hillsides or by slight elevation changes. Only about 1% of a watershed is stream channels. The smallest channels in a watershed have no tributaries and are called first-order streams. When two first-order streams join, they form a second-order stream. When two second-order channels join, a third-order stream is formed, and so on. First- and second-order channels are often small, steep or intermittent. Orders six or greater are larger rivers.

Channels change by erosion and deposition. Natural channels of rivers increase in size downstream as tributaries enter and add to the flow. A channel is neither straight nor uniform, yet its average size changes in a regular and progressive fashion. In upstream reaches, the channel tends to be steeper. Banks become lower as the width and depth increase in the lower reaches. More sand and silt are found downstream.

Streamflow

Besides the ordering system previously described, streams may be classified by the period of time during which flow occurs.

Perennial flow indicates a nearly year-round flow (90% or more) in a well-defined channel. Most higher order streams are perennial.

Intermittent flow generally occurs only during the wet season (50% of the time or less). Some places have streams and creeks with surface water flows that sink beneath the ground due to fissures and cavities in the underlying limestone formations and then re‑emerge some distance downstream. This is known as spatial intermittency.

Ephemeral flow generally occurs during and shortly after extreme precipitation or, in other areas of the world, during snowmelt conditions. Ephemeral channels are not well defined and are usually headwater or low-order (1–2) streams.

Natural groundwater discharge is the main contributor to streamflow during dry summer and fall months. Without groundwater discharge, many streams would dry up.

Factors Affecting Watersheds

The physical, chemical and biological makeup of a stream relates to surrounding physical features of the watershed and geologic origin. By analyzing these features, we can better understand stream-watershed relationships and predict effects of human influences on different stream types.

Climate
Land and water are linked directly by the water cycle. Solar energy drives this and other cycles in the watershed. A region’s source of water depends on its weather and climate. Water comes to a watershed in seasonal cycles, principally as rain or snow. In some areas, condensation and fogdrip contribute water. The seasonal patterns of precipitation and temperature variation control streamflow and water production.

Climate affects water loss from a watershed as well as provides water. In hot, dry or windy weather, evaporation loss from bare soil and from water surfaces is high. The same climatic influences that increase evaporation also increase transpiration from plants. Transpiration draws on soil moisture from a greater depth than evaporation because plant roots may reach into the available moisture supply. Transpiration is greatest during the growing season and least during cold weather when most plants are relatively dormant.

Area
The area of a watershed affects the amount of water produced. Generally, a large watershed receives more precipitation than a small one, although greater precipitation and runoff may occur on a smaller watershed in a moist climate than on a large watershed in an arid climate.
Shape and Slope

Shape and slope of a watershed and its drainage pattern influence surface runoff and seepage in the streams draining the watershed. The steeper the slope, the greater the possibility for rapid runoff and erosion. Plant cover is more difficult to establish, and infiltration of surface water is reduced on steep slopes. Soil depths and moisture-holding capacities are usually less on steep slopes, and plant growth rates are often slower.

Water moves downward, but not straight down — it follows the slope of the watershed. The water slowly filters through the sand, rocks and soil of an aquifer. It usually travels just a few inches each day. This slow movement keeps pollutants from being quickly dispersed and allows some of them to be intercepted and removed.

Orientation of the Land
Orientation of a watershed relative to the direction of storm movement also affects runoff and peak flows. A storm at the top of a watershed releases water which flows down the watershed. As the storm moves down through the watershed, rain continues to fall. The accumulation of upstream rain and continuing rain causes or increases flooding. A rainstorm moving up a watershed releases water so that runoff from the lower section passes its peak before runoff from the higher sections arrives. But the degree of flooding is also influenced by the size and geometry of the watershed and its physical features, such as pavement, wooded areas and wetlands.

Orientation of a watershed relative to sun position affects temperature, evaporation and transpiration. Soil moisture is more rapidly lost by evaporation and transpiration on steep slopes facing the sun. Watersheds sloping away from the sun are cooler, and evaporation and transpiration are less. Slopes exposed to the sun usually support different plants than those facing away from the sun. Orientation with regard to the prevailing winds acts in a similar fashion.

Soils and Geology
Soil is the outer, thinnest layer of the earth’s crust. It is composed of mineral particles of all sizes and varying amounts of organic materials.

Soils are of two types. Residual soils are those developed in place from underlying rock formations and surface plant cover. Characteristics of residual soils are closely related to the parent material from which they were formed. Transported soils include those transported by gravity, wind or water.

Soil often determines which plants will establish a protective vegetative cover. Plants also modify and develop soil. Plant roots create soil spaces and extract water and minerals through their roots. Plant litter adds organic matter to soil. Plant litter slows surface runoff and protects the soil surface from rainfall’s beating and the subsequent puddling effects.

Soil is the basic watershed resource. Careful management and protection are necessary to preserve its function and productivity.

Vegetative Cover
Grasses, forbs, shrubs and trees make up the major plant cover types which build up organic litter and affect soil development. They usually develop under differing climatic conditions and all are important to watershed management.

Plant cover benefits a watershed. The canopy intercepts rain and reduces the force with which it strikes the ground. The canopy and stems also reduce wind velocity.

When leaves and twigs fall, they produce litter, which decomposes and is eventually incorporated into the soil. Shade and mulch formed by plant litter reduce evaporation of soil moisture. Plant litter protects the soil surface, allows infiltration and slows down surface runoff.

Vegetation provides a physical barrier, slowing down the flow of runoff and providing more time for it to infiltrate the soil. Stems and roots lead water into the ground. Roots open up soil spaces for water retention and drainage and add organic materials to the soil. They also help bind or hold the soil in place.

Windbreaks of trees and shrubs protect crops and reduce moisture loss from evaporation. Grasses, trees and shrub stems along riverbanks trap sediments and floating debris during high-water flows. Roots bind and stabilize streambanks and slopes to reduce slides and slumps.

Management Considerations

Water quality is largely determined by the soils, vegetation and human activities in a watershed. Human activities include timber harvesting, livestock grazing, agriculture, recreation and urban or industrial development.

Timber
Timber harvesting opens the canopy cover and reduces plant cover density. Timber harvesting does not negatively affect a watershed if slope and soil are carefully considered and plant cover rapidly restored. Best management practices must be followed to ensure consideration of soil and water resources during timber harvest.

Agriculture
Domestic livestock tend to concentrate in specific areas when grazing. Concentrated grazing impacts plant cover and soil. Grass cover is improved by removing some annual growth, but productivity of the pasture is greatly reduced if overgrazing occurs. Excessive trampling by grazing animals can contribute to soil compaction, accelerated runoff and erosion problems.

Management of livestock and grazing wildlife species can enhance watershed values but is limited by the carrying capacities of the land and the forage species it supports. Management must consider timing, density and duration of animal use to capitalize on the positive aspects of grazing. Generally, recovery does not occur if vegetation is thinned to less than 70% of the natural cover. Without management practices such as reseeding, degradation will continue.

Animal waste management should be used in all livestock practices. Any areas where animals concentrate require waste management techniques. Concentration of animals increases the waste found in an area. As an area is intensely used by livestock, the soil is compacted and the land’s natural capacity to use the waste is reduced. The compacted soils accelerate runoff, which contains fecal material.

Crop production usually involves removing the original plant cover and tilling the soil for seedbed preparation. Crop cover is usually seasonal and less dense than natural cover, which affords less protection for the soil. Erosion by both wind and water may remove the finer and more fertile soil particles, reducing land productivity. Crop and grove production involves adding fertilizers and pesticides to crops which may run off into natural water systems. Agricultural operations based on careful appraisal of soil, slope and climatic conditions include erosion control and are compatible with watershed management.

Exotic Plants
An exotic plant is anything that is not native to a region. Exotic plants have had severe impacts on both aquatic systems and land systems. An increase in exotic plants has, in part, caused decreased streamflows. Many exotics compete more successfully than other vegetation for available moisture. This reduces groundcover and may cause increased runoff and less infiltration to groundwater storage. In addition, some exotics have high transpiration rates that leave less water for stream runoff as summer progresses.

Fire
Some of  natural ecosystems are based on a “fire ecology.” They are fire-dependent, meaning fire is used to maintain the vegetation necessary to the ecosystem. Fire triggers many plant species to reproduce or seed and eliminates pest or invasive species. Man uses fire as a conservation tool to maintain or restore altered ecosystems to a viable functioning system.

Fire can be beneficial to a watershed when it is carefully managed. It can reduce available fuel and prevent more-destructive fires which generally happen where fire has been suppressed for years. Fire thins understory seedlings that compete with larger trees for available moisture. Open-forest types, such as longleaf pine, are maintained by fire. Natural fires are beneficial if they do not burn too hot or if a large amount of fuel has not built up. Humans have suppressed natural fires in many areas, increasing the amount of fuel and the likelihood of “hot” fires burning out of control.

Fire is one of the most widespread agents affecting plant cover and can be either beneficial or destructive. Under certain conditions, fire can remove nearly all vegetation and organic litter and, in extreme cases, sterilize and change the chemistry of the surface soil. Burning converts organic materials in plant cover, litter and topsoil to gases, solubles and readily leached ashes that can make acid soils alkaline. Damage to soil varies, but it may take several seasons for soil conditions to return to normal.

Without a protective canopy and litter, the soil surface is rapidly puddled and sealed by the first rains. Infiltration is greatly reduced, making runoff and erosion more rapid. Debris-laden floods often occur within fire-denuded watersheds during only slightly abnormal rainfall. Most of the water falling on a burned landscape is lost by rapid runoff. Streams from burned watersheds at first carry a heavy load of salts dissolved from ashes, floating debris and sediments. Water quality soon returns to normal, except for sediment-laden high flows. These conditions may continue for several years until the plant cover becomes re-established on the watershed.

Mining
Mining requires opening the earth to remove mineral resources. It is done by stripping off the surface soil and rock layers or by drilling tunnels into the earth to reach minerals. With either method, quantities of waste material are left on the surrounding land. This waste material is subject to erosion and dissolution, adding to the dissolved sediment load of streams draining the mined area.

Surface changes include altered topography and drainage. Drainage from mined areas may contain toxic minerals or salts harmful to the aquatic habitat. Additives to extract the desired mineral — such as cyanide for gold — can enter the watershed if not properly managed. In most mining areas, phosphate, titanium and peat are mined; all of these types of mining produce waste materials. To prevent degradation of the watershed, waste material disposal must be carefully controlled and managed.

Development/Construction

Urban development involves

• Clearing, leveling and filling land surfaces
• Constructing buildings with impermeable roofs
• Paving roads and sidewalks with impervious materials
• Installing sewage disposal systems

Communication and transportation development includes roads, railroads, airports, power lines and pipelines. All of these involve disturbance of plant cover, soil and topography. Road and highway networks, with their impermeable paving and rapid drainage systems may radically change the runoff characteristics of their immediate area. They also require changing the natural topography and drainage and moving huge amounts of soil and rock. Often these networks are responsible for the discharge of sediments and may become the source of other water pollutants. Railroads and airports have similar effects. Power lines and pipelines require open paths through watersheds and access roads for construction and maintenance.

Human developments may greatly change infiltration and runoff, reduce recharge to underground water and increase runoff to produce rapidly fluctuating streamflows.

Air Pollution
Urban air pollution, especially photochemical smog caused by internal combustion gasoline engine emissions and industrial smokes, contributes to acid rain. This has an effect on vegetation, streams and lakes within watersheds, especially on the east coast and in Canada. The problem continues to grow, however, and no place is immune to the effects of acid rain.

Impoundments
Many dams or water control structures are built and operated for multiple purposes:

• To control floods
• To store water for irrigation or other consumptive use
• To regulate flow for navigation
• To provide power generation

Effects on streamflow and aquatic habitat are similar regardless of purpose. Impoundments, if shallow, allow water to warm and, if deep, preserve cooler water. As streamflow peaks are reduced and low flows increased, streamflow generally becomes more regular from season to season and year to year regardless of climatic variations. These changes in streamflow may affect migratory, endangered and threatened species, increase exotic invasives and otherwise alter the natural habitat of the region.

In many cases, reservoirs have added water-based recreation and new fisheries, although their construction may destroy stream habitat used by fish and other aquatic organisms. A watershed under good management — where water storage occurs in the soils and riparian areas — lessens the need for reservoirs, particularly small headwater impoundments.

Water is often seasonally diverted from impoundments and streams for irrigation in agricultural areas. This reduces streamflows during the warm growing season. Some water is returned to the stream by drainage from the irrigated fields. These return flows are warmed and may contain soil salts, fertilizers and pesticides leached from the fields.

In the past, mosquito impoundments were constructed to concentrate the mosquito larvae and facilitate spraying. Many of the impoundments still exist and have altered the water patterns of local areas. Some impoundments are still sprayed on a regular basis during the warmer seasons which then makes them a potential source of pesticide pollution. Blanket spraying for mosquitoes, whether done at impoundments or throughout a region, can alter the natural food chain for an area. Most pesticides are not selective for just mosquitoes but also eliminate beneficial insects as well.

Management Techniques

The objective of managing a watershed is to maintain useful vegetative cover and soil characteristics beneficial for good water quality. When the non-renewable soil resource is protected and maintained in good condition, the dependent renewable resources, wildlife habitat and recreational opportunities, can be supported.

Timber, forage, minerals, food and wildlife represent important considerations. Problems arise when development and use of these resources conflict with the primary objective of regulating water yield and maintaining water quality and watershed integrity. These must be considered as part of watershed management. Their use and development must be integrated as part of a management system that produces and protects water supplies.

Land ownership is the principal institutional control of watersheds. A private individual or public management agency may be free to apply whatever measures are believed necessary or desirable on their own land. They may regulate access and prevent use and development of associated resources.

Ownership of most watersheds is mixed between public and private landowners. Most watersheds are used and developed to take advantage of all resources available. It is in these multiple-use, multi-owned watersheds that management faces the most serious conflicts and challenges.

It is necessary to attain balanced use and development with the least disruption of the water resource. Watershed users need to be aware that private actions have public consequences on water quality and quantity.

Legislation and government rules and policies also provide controls that can aid water resource management. These laws may include

• Land use planning
• Zoning
• Permitted and prohibited land uses or types of development
• Restrictions on water use
• Limitations and/or requirements on development
• Pollution control
• Minimum flows and levels
• Special designations such as LGOs

Summary

Rivers, hillsides, soils, forests and bottomlands are all part of one integrated system. Hillside shape and slope control the rate or energy of water flow. All biotic elements in the watershed interact with and modify the energy flow through the system. So it follows that the shape of the watershed is a function of what lives there. The combination of climatic conditions, soil types, topography, vegetative cover and drainage system define the particular character of each watershed.

Rivers do not stop at state lines. The effects of natural and human processes in a watershed are focused at its outlet, wherever that may be, even if it crosses another state’s or country’s borders. Each watershed is a part of a larger watershed whose downstream portion may suffer from upstream influences.

Impacts on water quality and quantity (from private actions) occur by either reducing or increasing the levels of chemical compounds and flow volume beyond the watershed’s ability to absorb (in the case of increase) or meet its needs (in the case of reduction).