Imagine a glacier or snowfield melting on a summer day. Now imagine each drop of water becoming part of a mountain stream. The stream plummets down the mountain slope, merges with another stream, joins a river, which merges with yet an even larger river. Eventually some of the drops of water reach the ocean or some other body of water. The water drops become part of a network of streams. The network of streams is called a drainage system, with the area drained by the river system called a drainage basin or watershed.
Drainage basins vary in size. Basins can be drained by a single river or stream, or can consist of a huge network of rivers, including thousands of streams and creeks. The Columbia River Basin is an example of a very extensive drainage basin, incorporating 259,000 square miles. The vast area drained by the Columbia extends from British Columbia, Canada and includes the majority of the states of Washington and Oregon, parts of Montana, and all of Idaho, with some tributaries initiating in Nevada, Utah, and Wyoming. Impressive!! Large rivers, such as the Snake, Salmon, and Willamette Rivers, are part of the huge Columbia River network, yet each has its own large drainage basin as well.
The Columbia River Drainage Basin is shown in the light cream color. Smaller drainage basins that are part of the vast Columbia River system are the Snake River system (purple dotted outline), Salmon River system (brown dotted outline), and Willamette River system (dark red dotted outline). Note that the Salmon River system is part of the Snake River system, which is part of the Columbia River system.
Obviously, a stream or river consists of flowing water (at least part of the year anyway). But, the water flow is not consistent and varies throughout the year. The stream channel changes, adjusting to flow differences. Important variables of stream flow and stream channels include gradient, discharge, velocity, and width and depth (cross-sectional area) of the stream.
Gradient is
the stream's slope or the vertical drop of the stream channel
over a specific horizontal distance. It is typically measured
in feet/mile (meters/kilometers). A stream flowing down a mountain slope will
have a steep gradient and a narrow channel, while a stream flowing across a
flat plain will have a gentle gradient.
Ryder Run (shown in the figure) is approximately 1.25 miles long. The change in elevation (measured from the R in Ryder Run to the river) is 2280 feet – 1780 feet = 500 feet. Therefore, the gradient for Ryder Run is 500/1.25 = 400 feet/mile.
Discharge is the volume of water passing a point over a period of time, typically measured in cubic feet per second (cfs) or cubic meters per second (cms). Discharge is based on the cross-sectional area (multiply width by depth) of the stream and the velocity of the water. The width of the stream is the horizontal distance measured from bank to bank. Depth of the stream is the vertical distance from the water's surface to the channel bottom (or, streambed). Measurements are taken from several points across the width of the stream, then averaged to determine the stream's depth.
The
equation defining discharge is:Changing one of the parameters in the equation inevitably changes the discharge as well. A change in discharge results in altering the shape of the stream channel.
Most stream channels can fit into three main categories: straight, meandering, and braided. Straight channels can have steep or gentle gradients, as can braided channels. Meandering channels are typically located in flat or nearly-flat valleys or plains. The topographic area adjacent to a stream in a valley with little or no slope is called a floodplain.
Straight
channelsStraight channels form in response to steep gradients, channels of resistant rock, and linear features such as fractures and joints. Steep gradients enable water to flow in the straightest path downhill. Resistant rock inhibits sideways (lateral) erosion of the stream channel. Erosion of linear fractures and joints allows easy straight paths for streams to follow. Channel erosion of a linear channel is generally downward into the underlying strata.
Meandering channels have a circuitous-shape. Each individual rounded bend is called a meander. Over time, meandering streams migrate back and forth across valleys, eroding and depositing sediment as they migrate. Channel erosion and deposition is lateral. Most erosion occurs on the outside of the bend, which is called the cutbank. Deposition occurs on the inside of the bend, which is called the point bar.
The cutbank and point bar are each in the same locations in both the illustration and photo.
Braided channels occur when the sediment load transported by a stream is greater than the stream's carrying capacity. In order to maintain equilibrium (based on discharge [Q=VWD] and gradient), a stream must make some kind of change. Dumping excess material is one easy way! Deposits form gravel bars, inchannel bars, and islands that divide the flowing water into a number of channels.
Now that you're "thoroughly" familiar with stream discharge and channel shapes, you must be wondering what the point of all this is. So, why is discharge important?? Water levels of streams and rivers fluctuate! Not just year to year or month to month, but daily!!! And, you can monitor such changes. How? Stream discharge is measured at gaging stations set up at several locations along a stream. Each gaging station contains a water level indicator. Fluctuations recorded by the water level indicator correspond to changes in discharge.
Select the "REAL TIME" button on this page. A map of the United States should come up. Pick a state the map. Choose 2 gaging stations within that state.
Compare: peak flow for 1990-1996, current conditions data, and historical streamflow daily value. Include station number, station name, county, state, basin name, latitude, longitude, and drainage area.
How has the stream flow changed at each site? How are the sites different physiographically? How do the peak and low flow periods differ for these two sites?
What
causes flooding? The answer is quite simple. Flooding occurs when there is excessive
stream discharge (remember Q=VWD?). Flooding occurs when channels are filled
beyond their capacity and results in excess water spilling out of the channel
onto the adjacent floodplain. Flooding is a natural-occurring process and is
another method that a stream 'uses' to maintain its equilibrium. Flooding can
be the result of any of the following processes: excess precipitation, rapid
snowmelt, increased runoff, steep slopes, storm surges, tsunami, dam failure,
and urban development.
How large could a flood be in your area? How often could a flood occur? These are questions that you need to ask to evaluate possible flood conditions. Magnitude and frequency are terms that are used to describe flood conditions. Minor flooding of an area that occurs annually has a high frequency, whereas catastrophic flooding of an area occurs very, very infrequently, therefore has a low frequency. Major floods are the result of relatively infrequent combinations of weather and hydraulic conditions. They are considered high-magnitude floods.
Can you predict how often a certain size flood will occur? Provided you have enough flood discharge data, you could statistically estimate the probability. Determining the likelihood of a catastrophic flood requires a large database of information. The longer flood records are collected, the better the estimation will be. The probability of a 20-year flood occurring next year is 5% (based on 1 ÷ 20). The following describes the procedure for determining recurrence intervals over a 20-year period.
Flood discharge data has been collected for N (in this case- 20) years of record for the drainage basin of interest. Magnitude of discharge is ranked from highest to lowest on an annual basis. Year with the highest magnitude is ranked as M = 1, while the year with the lowest magnitude is ranked as M = 20 (M = N).
Therefore, if your R-value = 20, the recurrence interval would be referred to as a 20-year flood. But, if a 20-year flood did occur, don’t think it would be another 20 years before another flood that size would occur! Flood recurrence values are just estimations, values averaged over a period of time. So under the right conditions, another 20-year flood could happen the very next year!!!
Determine what locations in the United States may be susceptible to flooding on the day you check. This assessment page can change daily therefore you need to record the date you did this particular assignment.
1. Using the USA map, list the states where 2 inches or less of rain over a 3-hour period could result in a floodthreat.
2. Now you will forecast which 2 states should expect the maximum precipitation today.
3. Next, determine which states may actually have a floodrisk. Include whether it is a flood warning, a flood watch, or both for each state containing floodrisks.
Floods have different characteristics! Upstream flooding occurs in the upper regions of a drainage basin. Rainfall is very intense, but short-lived. And, effects are local. Downstream flooding is related to larger-scale weather events. Flooding covers a much greater area of a drainage basin, with many tributaries contributing floodwater. Flood levels may last for days!!! Property damage can be extensive!!! Therefore, downstream flooding is generally more devastating.
Urbanization has had a major impact on river systems! Many towns and cities are located next to major rivers and their tributaries. Many of these cities are sitting smack in the middle of flood plains!! Highways, streets, parking lots, sidewalks, and buildings now cover large areas of ground surface-areas that "use to" absorb excess rain water, vegetated areas that slowed a stream's discharge rate.
Now the hard-covered surfaces act as conduits for excess stormwater to rapidly travel over. The time between the peak rainfall and peak discharge (called lag time) decreased drastically!! The result?? Increased flooding!!! And, if the lag time is short enough, "flash flooding" may occur. Major disaster!
Flash floods can be extremely devastating with little or no warning. Read the following 3 articles for first had experiences of flash floods.
Read Open File 93-641 for more information.
Read about the Big Thompson Flood
Read a personal account of the Little Wild Horse Flash Flood
(These 3 articles are printed from the internet in a pdf format so that if the websites cease to exist the articles are still available for reading for this course.)
Urbanization requires an adjustment to periodic flooding. Flood mitigation minimizes the effects of flooding. Creating natural greenway corridors along riverbanks is one successful method. Greenways are used for recreational purposes during the "dry" season, but act as basins during flooding. Another method that allows natural flooding along rivers is restoration of riparian zones.
Flood control is necessary in highly urbanized areas. Therefore, such locations require more stringent measures such as channelization, levees, dams, diversion channels and floodwalls to minimize flood damage. Flood-hazard maps are designed to reduce u r b a n s p r a w l in flood-prone areas. This, in turn, greatly decreases flood-damaging effects.
http://www.fema.gov/hazards/floods/
http://www.uidaho.edu/disaster/
http://www.disastercenter.com/idaho/idaho.htm
Assume there is going to be a horrendous squall in your town tonight, one large enough to result in flooding. What should you do to be prepared before the flood occurs? What about during the flood? And, after??
Describe what you would (and should) do in each of the above circumstances.
Back to ISU Geosciences Web Courses Home
