It's equally interesting to think that one drop of water released so far inland will eventually make its way to the lake.  We expect this will happen because water runs downhill under the notorious force of gravity. When we trace the path of all the streams flowing into each consecutive watercourse, we notice that the river drains water from an enormous area or basin.  Not only does the river capture the flow of all the tributaries, but it also catches the water that falls on the adjoining land that drains toward the streams.  As water descends through the landscape, it picks up sediment, nutrients, and organic matter along its route.

The highest points of land adjacent to the river define the drainage boundary.  We refer to this area as the river's watershed.  A subwatershed, as the name implies, encompasses all of the lands that are drained by a tributary to the main watercourse.  Of course, there are other names that describe the same particulars, such as drainage basin or catchment area, but we prefer to use watershed since it is the most generally recognized term.

Picture a watershed as a three-dimensional landscape that is a direct product of local climate and geography.   Each watershed has a natural character that is expressed by the size of the landscape it drains, the surficial soils and topography that the water passes through, and the climate that produces the volume of water available for groundwater recharge, discharge, and surface transport.  This character has evolved through more than ten thousand years of precipitation, erosion, and deposition.

The geology of the watershed combined with climate dictate the volume of water available to the stream as either baseflow or surface run-off.  Surface water, in combination with groundwater recharge and discharge, determine the hydrology of the watershed.  Flat or shallow sloped land will tend to infiltrate more precipitation than steep slopes of the same soil type.  In porous, sandy soils, precipitation quickly soaks into the ground, recharging the groundwater table.  This same groundwater appears downstream as cold discharge seeps that contribute to the baseflow of the stream.  Tight soils such as clays and silts are less capable of recharging groundwater, and instead, convey surface water toward the collection system of tributary streams.  Exposed bedrock, much like asphalt and roof tops, sheds water even faster.  Some precipitation that soaks into the ground is soon lost to the atmosphere by evaporation or through evapotranspiration (sweat) from plants.  This cycling through precipitation, evaporation, transpiration, recharge and discharge is a natural circulation system of water between the earth and the atmosphere.  Each of these water pathways is interrelated within the hydrologic cycle.

The character of the soils in the watershed influences the quality of the surface water entering the river.  Water transports dissolved and solid materials from the land to the stream.  Alkaline soils such as gravels, sands, and limestone bedrock produce alkaline surface water.  Granite bedrock produces more acidic water.  Asphalt roadways can contaminate run-off with oils, salt and heavy metals.  Exposed erodable soils such as silts, clays, and sands eventually drain into streams, which causes an increase in turbidity and sediment bedload.  In contrast, forested watersheds move woody debris and organic matter such as leaves and needles that provide nutrients, forage, and cover for aquatic life.  The naturally occurring dissolved and solid materials transported by streams make up the basic elements needed to sustain living organisms.

The climate and geology also dictate the form of the stream and its floodplain as it passes through its valley.  As water moves downhill, collecting sediment along the way, the size and volume of sediment combined with the slope of the land and volume of water create a dynamic stream.  This stream is linked to its floodplain.  Steep rock based streams may be a series of rapids cascading downward moving water and sediment in a linear path.  Shallow slope streams, carrying sediment in a channel with predominantly sands and gravels, may meander back and forth through the valley.  Frequent flooding by storms and spring snowmelt help form the floodplain of the river, which acts as an area of flood relief and sediment deposition.  The bankfull channel, which occurs once every one to two years in an undeveloped watershed, defines the physical characteristics of the river. The term fluvial geomorphology is the technical term that captures these natural channel processes.