Drainage Basins

• Geomorphologists and hydrologists often view streams as being part of drainage basins. A drainage basin is the topographic region from which a stream receives runoff, throughflow, and groundwater flow. Drainage basins are divided from each other by topographic barriers called a watershed .A watershed represents all of the stream tributaries that flow to some location along the stream channel.

The following image shows the nested nature of drainage basins as determined from a topographic map sheet. The red lines describe the watersheds for the drainage basins of first order streams. The yellow lines define the watersheds for two drainage basins from locations further upstream.

• Drainage basins are commonly viewed by scientists as being open systems. Inputs to these systems include precipitation, snow melt, and sediment. Drainage basins lose water and sediment through evaporation, deposition, and streamflow. A number of factors influence input, output, and transport of sediment and water in a drainage basin. Such factors include topography, soil type, bedrock type, climate, and vegetation cover. These factors also influence the nature of the pattern of stream channels

Hydrographs:A hydrograph may be used to show how the water flow in a drainage

basin (particularly river runoff) responds to a period of rain. • This type of hydrograph is known as a

storm or flood hydrograph and it is generally drawn with two vertical axes. One is used to plot a line graph showing the discharge of a river in cumecs (cubic metres per second) at a given point over a period of time. The second is used to plot a bar graph of the rainfall event which precedes the changes in discharge.

• The scale on the horizontal axis is usually in hours/days and this allows both the rain event to be recorded and the subsequent changes in river discharge to be plotted

Shape• The shape of the hydrograph varies according to a number of controlling

factors in the drainage basin but it will generally include the following features.

• The baseflow of the river represents the normal day to day discharge of the river and is the consequence of groundwater seeping into the river channel. The rising limb of the hydrograph represents the rapid increase in resulting from rainfall causing surface runoff and then later throughflow. Peak discharge occurs when the river reaches its highest level. The time difference between the peak of the rain event and the peak discharge is known as the lag time or basin lag. The falling limb (or recession limb as it is sometimes known) is when discharge decreases and the river’s level falls. It has a gentler gradient than the rising limb as most overland flow has now been discharged and it is mainly throughflow which is making up the river water.

Influence of basin shape• A number of factors (known as

drainage basin controls) influence the way in which a river responds to precipitation and have an effect on the shape of the hydrograph.

• The size, shape and relief of the basin are important controls. Water takes longer to reach the trunk stream in a large, round basin than in does in a small, narrow one

Influence of steepness• Where gradients are steep,

water runs off faster, reaches the river more quickly and causes a steep rising limb. Prolonged heavy rain causes more overland flow than light drizzly rain

Differences• Areas of permeable rocks and

soil allow more infiltration and so less surface run off.

Do you understand?

The following situations will affect the storm hydrograph by producing either a) a short lag time and steep rising limb or b) a long lag time and gentle rising limb

• • a rainstorm in an area underlain by granite • a long period of 'drizzly' rain during summer • a snowstorm in the Highlands of Scotland • a thunderstorm in a city • a rain event in an area which is heavily forested • a period of showers in an area where there is much newly ploughed, fairly level, arable land

• For each of the situations listed above, select either (a) or (b) and give reasons for your choice