All Land on Earth Is a Watershed. Humans
Total Page：16
File Type：pdf, Size：1020Kb
Watersheds “The study of rivers is not a matter of rivers, but of the human heart.” 4 — Tanaka Shozo ll land on earth is a watershed. Humans or a river basin as large as that of the Columbia and their activities play an important and River. A puddle even has its own watershed. A essential role in watersheds, yet few Within a large watershed tributaries form people understand them. Still fewer know how a smaller watersheds called sub-basins. Each watershed works or can describe the boundaries tributary contributes to overall streamflow for the of the ones in which they live. entire basin. Oregon has 20 major river basins A watershed is often called a drainage basin. (see Figure 4.) It is the land area drained by a network of chan- All watersheds have an aquatic (or water) nels, called tributaries, that increase in size as area, a riparian area, and an upland area. Aquatic the amount of water, sediment, and dissolved areas include standing waters like ponds, lakes, materials they must carry increases. Each water- wetlands, bogs and running surface waters such shed is an interconnected land-water system that as streams and rivers. The corridor of vegetation conveys water to its outlet—a larger stream, an next to and influencing the aquatic area is called inland lake, a wetland, an estuary, or the ocean. the riparian area. A watershed may be the drainage area sur- The point where two watersheds meet is rounding a lake that has no surface outlet, such as called a divide. Connecting the divide with the Malheur and Harney Lakes in southeast Oregon valley or lowland areas below are the hill slopes or uplands. Events in the uplands ultimately Figure 4. Oregon River Basins North Vocabulary Coast Columbia Umatilla Grande aquifers perennial Sandy Hood Ronde baseflow plant associations climate radial drainage Willamette John Day Powder dendritic drainage residual soils deposition riparian area Mid Snake Coast divide streamflow hydrograph Deschutes Malheur ephemeral sub-basins erosion sublimation first-order streams transported soils South Umpqua Goose and forage trellis drainage Coast Summer Lakes Owyhee gradient tributaries Malheur Klamath Lake intermittent uplands Chetco Rogue leaching water equivalent parallel drainage watershed Oregon Department of Fish & Wildlife Watersheds • 33 affect the capture of water on the surface of Headwaters the land, storage and movement of water below the surface, and release of water to Stream Sub-basin riparian and aquatic areas. Each stream in a watershed is an ever- changing open-water system. It carves through Ridge or valleys, collects water and sediments, and con- watershed veys the surface runoff generated by rainfall, divide snowmelt, or groundwater discharge to the estu- aries and oceans. The shape and pattern of a Uplands stream is a result of the land it is cutting and the Present Floodplain river sediment it must carry. channel Each of us has a “watershed address,” which describes our basic relationship with a water- shed. One part of our address is our location. We all live in topographic watersheds—areas drained A watershed is almost like a domicile, a mini- by a common stream. When a raindrop falls on biosphere, with halls of hills and mountains, a floor the roof of our house, where is it going? What of river or lake, and a roof of rain clouds. Adapted creeks or rivers will carry it toward the sea? from Co-Evolution Quarterly, Winter 1976/77. Some people also live in engineered water- sheds, which may not follow topographic lines. When we turn on the faucet in the kitchen sink, Area what watershed did that water come from? When The area of a watershed affects the amount of the water runs down the drain, what watershed is water that flows from the river or stream that it going to? For example, while rainwater in drains it. Generally, with similar climates large much of the Portland Metro area flows into the watersheds receive more precipitation than small Willamette River, much of Portland’s domestic ones. Greater precipitation and runoff may occur water supply is piped from the nearby Bull Run on a smaller watershed in a moist climate than on Watershed, a watershed that flows toward the a large watershed in an arid climate. Columbia River. In this way, one watershed is artificially connected to several other watersheds Shape and slope at once. The watershed of surface flow, the Shape and slope of a watershed and its drainage watershed where domestic water originates, and pattern influence surface runoff and seepage in the watershed where wastewater goes are all streams draining the watershed. The steeper the connected. This means Portland residents live in slope, the greater the possibility for rapid runoff one watershed and drink water from another, and erosion. Plant cover is more difficult to while their wastewater may affect their “home establish and infiltration of surface water is watershed” and others. reduced on steep slopes. Physical features of a Orientation watershed Orientation of a watershed in relation to the Rain, snow, wind, ice, and temperature variations direction that storms move across it also affects are all agents of erosion in a watershed. The runoff and peak flows. A rainstorm moving up a erosional effects of surface water create stream watershed from the mouth releases water in such channels. As streams carve their way through a a way that runoff from the lower section has watershed, they are responsible for most of the passed its peak before runoff from the higher “topographic identity” of a watershed. sections has arrived. A storm starting at the top 34 • The Stream Scene: Watersheds, Wildlife and People Oregon Department of Fish & Wildlife and moving down a watershed can reverse the Figure 5. Stream Orders process. Orientation of a watershed relative to sun 1 1 position affects temperature, evaporation, and 1 transpiration. Soil moisture is more rapidly lost 2 2 1 by evaporation and transpiration on steep slopes 1 3 facing the sun. Watersheds sloping away from 2 3 the sun are cooler, and evaporation and transpira- 1 3 2 tion are less. Slopes exposed to the sun usually 1 3 1 support different plants than those facing away 4 from the sun. Orientation to prevailing winds has similar effects. 4 1 3 5 Drainage patterns 2 1 2 Viewed from above, the tributaries of each river system create a distinct pattern. Geology, topog- 5 raphy, and climate are responsible for this pat- tern. Regions with parallel valleys formed by the folding of the earth’s surface have a parallel age pattern of the individual sub-basins formed drainage pattern. Where the geology is sedimen- by these streams have a dendritic pattern. tary rock, fault lines may create a drainage pat- tern where streams flow parallel to each other Stream orders and tributaries join at nearly right angles in a In most cases, a watershed system is almost trellis drainage pattern. entirely hillsides, called uplands. Only about one In the Pacific Northwest two of the most percent of a watershed is stream channels. The common patterns are radial drainage and den- smallest channels in a watershed have no tribu- dritic (treelike) drainage. When streams drain a taries and are called first-order streams. When central high point, such as a mountain top, they two first-order streams join, they form a second- create a pattern similar to the spokes on a wheel order stream. When two second-order channels radiating out from the central hub. This is radial join, a third-order stream is formed, and so on drainage. (Figure 5). First- and second-order channels are The branching tributaries of a river may also often small, steep, or intermittent. Orders six or create a pattern similar to the branches of a tree. greater are larger rivers. This is dendritic drainage. Both types may occur Channels change by erosion and deposition. within the same watershed. For example, the Natural channels of rivers increase in size down- radial pattern of streams that drain Mount Hood stream as tributaries enter and add to the flow. are all within the Columbia Basin, but the drain- Radial Drainage Dendritic Drainage Parallel Drainage Trellis Drainage Oregon Department of Fish & Wildlife Watersheds • 35 A channel is neither straight nor uniform, yet its is the main contributor to streamflow during dry average size changes in a regular and progressive summer and fall months. Without baseflow, fashion. In upstream reaches, the channel tends many streams would dry up. to be steeper. Gradient decreases downstream as Pumping water from an aquifer for industrial, width and depth increase. The size of sediments irrigation, or domestic use reduces the aquifer’s tends to decrease, often from boulders in the hilly volume. Unless withdrawals are modified or or mountainous upstream portions, to cobbles or recharge increased, the aquifer will eventually be gravels in middle reaches. More sand or silt are depleted. A drained aquifer can collapse from the found downstream. In some cases, large floods settling of the overlying lands. cause new channels to form, leaving once-pro- Collapsed underground aquifers no longer ductive streams dry and barren. have as much capacity to accept and hold water. Recharge is difficult, volume is less, and yields Streamflow types Besides the ordering system previously de- scribed, streams may be classified by how much Land and water are linked of the year they have flowing water. directly by the water cycle. • Perennial flow indicates a nearly year- round flow (90 percent or more) in a well- defined channel. Most higher order streams are perennial. are considerably reduced. Springs once fed from • Intermittent flow generally occurs only the water table also dry up.