closed basinA watershed with no way out to the ocean. Its water gathers in lakes and swamps and leaves by evaporating. The Great Basin, which spans nearly all of Nevada, is the largest such area in North America. Scientists call it an endorheic basin.
continental divideA divide whose two sides drain to different oceans. The Continental Divide of the Americas follows the crest of the Rocky Mountains. It separates rivers bound for the Pacific from rivers bound for the Atlantic and the Arctic.
drainage basinWhat a watershed is called outside North America. Catchment is a third name for the same thing. Outside North America the word watershed keeps its first sense: the line of high ground between two basins.
drainage divideThe ridge line where water parts to flow two ways. On rugged land the divide runs along ridges. On flat or marshy land it can be hard to see.
estuaryA tide-washed bay at the end of a river. The largest in the United States is Chesapeake Bay. Its watershed covers about 64,000 square miles in six states and Washington, D.C.
Show all 36 terms (31 more)
Parts of a watershed, continued
headwatersThe small streams where a river system begins. A river with many tributaries has many headwaters. Headwater streams get most of their water straight from rain and snow.
hydrologic unitA mapped drainage area, nested inside larger ones. The United States Geological Survey maps the whole country as hydrologic units, each with a code number. Larger units hold many smaller ones.
main stemThe principal river that the side streams all join. It is also called the trunk. Deciding which headwater is the true start of a main stem is not always simple.
outletThe one point where a watershed's water flows out. The outlet can be the mouth of a bay, the outflow of a reservoir or any chosen point along a stream. Choose another outlet and the watershed is a different one.
stream orderA number that ranks a stream by its branching. A stream with no tributaries is first order. Where two first-order streams meet, a second-order stream begins.
tributaryA stream that adds its water to a larger stream. The Missouri, the Ohio and the Arkansas are major tributaries of the Mississippi.
watershedThe area that funnels its rain to a single outlet. Every piece of land is part of one. A watershed can be as small as the land around a pond or cover thousands of square miles.
Water on the move 12
aquiferA body of saturated ground that can feed a well. It can be sand, gravel or rock with cracks and pores. Water that soaks down from the surface to refill it is called recharge.
baseflowThe steady streamflow that groundwater keeps going. It keeps a stream running between rains. It is also called drought flow or fair-weather runoff.
evapotranspirationWater sent back to the air by soil and plants. The word joins evaporation, from soil and open water, with transpiration, the water that plants give off.
groundwaterThe water of the soaked zone under the water table. Groundwater moves slowly. It may stay in storage for a long time before it comes back to the surface in a spring or a stream.
infiltrationThe seeping of rain down into the ground. Some of the water stays in the upper soil and is taken up by plants. Some moves deeper and refills aquifers.
precipitationThe moisture that weather delivers to the land. Rain, snow, sleet and hail are forms of it. The amount that falls on a watershed is by far the greatest factor in how much water its streams carry.
riparian zoneThe belt of streamside land and its plants. Plants that grow there are called riparian vegetation. A planted strip left beside a stream to shade and protect it is a riparian buffer.
runoffRain or meltwater sheeting downhill across the land. It happens when rain arrives faster than the soil can take it in, or when the soil is already full of water.
surface waterWater on top of the land, as in rivers and lakes. Lakes, streams, reservoirs and wetlands are all surface water. Most of it began as precipitation.
water budgetAn account of water entering and leaving a watershed. Water comes in as precipitation. It leaves as evaporation, as recharge to aquifers and as streamflow at the bottom of the watershed.
water tableThe upper limit of ground that is soaked with water. Below it every pore and crack in the ground is full of water. Above it the ground holds air as well.
wetlandGround wet enough, long enough, to grow marsh plants. Wetlands lie between open water and dry land. They clean the water that passes through them and help to control floods.
People and water quality 12
algal bloomA rapid, dense growth of algae in a lake or bay. A bloom often discolors the water. Extra nutrients in the water act like fertilizer for the algae.
impervious surfacePavement, roofs and other cover that sheds water. Rain cannot soak through it, so it runs off fast. Roads, parking lots and rooftops are impervious, and so is soil packed hard by building.
land useWhat people do with a piece of land. Forest, cropland, grassland and settlements are kinds of land use. Land cover is a closely related term.
nonpoint source pollutionPollution washed in from many scattered places. Rain and snowmelt gather it as they move over and through the ground, so it is hard to trace back to any one source.
nutrient pollutionToo much nitrogen and phosphorus entering the water. The nutrients run off from fertilized lawns, gardens and farm fields and from pet and animal waste.
point source pollutionPollution that enters water from one known place. A factory or a sewage treatment plant that discharges into a river is a common kind of point source.
pollutantA contaminant, such as oil washed off a street. Flowing water picks pollutants up as it crosses the land: grease, fertilizer, pesticides, bacteria and loose soil.
rain gardenA planted hollow made to catch and soak up runoff. Rain from a roof, a driveway or a street is led into it and sinks into the ground there. It is planted with grasses and flowering plants.
storm drainAn opening and pipe that carry street runoff away. Many storm drains empty straight into a river or stream, so whatever is poured into one ends up in that water.
stormwaterStorm rain pouring off streets, roofs and pavement. Most of it reaches streams and rivers without being treated.
turbidityHow cloudy water is with floating particles. It is a key test of water quality. Rain makes a creek turbid, turning it brown with sediment.
water qualityThe condition of water, measured against a standard. Clear water with plenty of dissolved oxygen is one sign of good quality in a stream or bay.
This watershed vocabulary worksheet lists watershed, drainage basin, drainage divide, continental divide, headwaters, tributary, main stem, outlet, estuary and three more terms beside their meanings in another order. The answer key is the last page.
Read the full guide
Water on the move
Set opens on Parts of a watershed. The second set has precipitation, runoff, infiltration, groundwater, aquifer, water table, evapotranspiration, surface water, water budget, wetland, riparian zone and baseflow.
People and water quality
The third set has land use, impervious surface, stormwater, storm drain, pollutant, point source pollution, nonpoint source pollution, nutrient pollution, algal bloom, turbidity, rain garden and water quality.
Evaporation, condensation and the other steps of the water cycle are on the water cycle word search.
Questions and answers
What is the largest watershed in the United States?
The Mississippi River watershed. It drains about 1.15 million square miles, from the Rocky Mountains to the Appalachians, in more than 30 states and two Canadian provinces.
Why are watersheds important?
Because whatever happens on the land shows up in the water. The flow and the quality of a river depend on everything in its watershed above that point: the rain and the soil, and the farms, streets and towns.
How does a watershed work?
Rain and melted snow run downhill. High ground divides the water, and on each side rills, creeks and streams gather it toward one river and one outlet. Some of it soaks in on the way and reaches the stream later as groundwater.