Watersheds and Hydrology

An Introduction

Water connects our landscapes.  As the rain falls on mountain slopes, it filters downhill into small streams, eventually feeding into larger tributaries and downstream river basins.  Some precipitation also filters into soils and makes its way below the surface into subterranean flows and groundwater aquifers.  Smaller amounts of the precipitation will evaporate back into the air or be taken up by plants.  

Understanding how water moves across and through the land and maintaining these connections are vital to proper management of fishery resources.  When we interrupt flows, by dams, water diversions, or other activities, we disrupt nutrient cycles, pollution loading, and hinder movement of fish, invertebrates, spawning gravels and other sediment.  Channelization serves to separate rivers from their floodplains.  Hydrology can be disrupted by changes on land as well as in streams.  Water flows are disrupted when terrestrial landscapes are fragmented by land use changes, roads, and other developments.

Within a watershed, or catchment, the drainage network can be divided by stream orders.  The smallest, headwater streams are first-order streams.  Such streams may be intermittent, that is, flowing only during spring or other periods of high runoff.  As two first-order streams flow together, they form a second-order stream.  Unlike headwater streams, these streams usually are perennial and flow year round.  This process of smaller streams building into larger streams repeats itself until flows reach the larger, fourth-order streams or rivers.      

 Horton stream order system

Watersheds themselves are nested in a hierarchy.  Smaller subwatersheds -- sometimes referred to as 6th level hydrologic units - are nested within larger watersheds, which are in turn nested within subbasins, and then within larger river basins.  Some watersheds can be divided into 5 or 6 subwatersheds.  Similarly, some river basins can be divided into 5 or 6 subbasins.  Each catchment collects, stores, and feeds water into the next larger hydrologic unit.  A subwatershed may be as small as 10,000 acres in size, whereas a subbasin might cover hundreds of square miles.  (see EPA's Find Your Watershed [1])

The primary function of watersheds is the collection, storage, and release of waters and sediment from upstream and upslope into downstream areas.  Many variables control this function.  Some variables are innate to the watershed, such as size, plant communities, precipitation patterns, watershed orientation, and topography.  Other variables, including dams and other hydrologic barriers, roads, and changes in plant communities, are dictated by human uses.  As more impervious surfaces are constructed within a watershed, including buildings, roads and parking lots, runoff is accelerated.  Highly developed watersheds will exhibit earlier and higher peaks in runoff when compared to watersheds with more natural vegetation that slows runoff.  Spring runoff and flooding can be more severe in urbanized watersheds or watersheds with high road densities.  Higher runoff often equates to accelerated rates of erosion and stream sedimentation.

Vegetative cover also is critical in controlling water and sediment flow rates.  Perhaps the most important vegetative features, relative to flows, are found in riparian zones along streams.  Increases in riparian plant communities can retard erosion and greatly benefit stream systems.  Large wood, a critical component of stream health, also is contributed by riparian habitats.  For these reasons, riparian zones often are critical targets for proper watershed management.

References:
Leopold, L.  1994.  A view of the river. Harvard University Press, Cambridge, Massachusetts.

Maxwell, J.R., C.J. Edwards, M.E. Jensen, S.J. Paustian, H. Parrott, and D.M. Hill.  1995.  A hierarchical framework of aquatic ecological units in North America (Nearctic Zone). USDA Forest Service General Technical Report GTR-NC-176, St. Paul, Minnesota.

Links:
EPA Watershed Handbook [2]
USGS Glossary of River Terms [3]