The Watershed Thread
This is the second installment of The Watershed Thread, a new educational campaign about watersheds from the North Dakota Department of Water Resources.
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The General Zones of a Watershed Basin | |
A watershed basin is the land area that water flows over or under on its way to a river, including the land drained by its tributaries, which are smaller rivers or streams that flow into the larger river. To better understand how watersheds function, it’s easiest to divide the basin into three sections: upper, mid, and lower. Understanding, of course, that all watersheds are unique. As water moves through each section, the river gets wider, and the volume of water increases along with the amount of sediment and debris being transported to its endpoint which could be a reservoir, lake, or ocean. Let’s look at our example watershed and the characteristics that can be observed in each section. | |
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Upper Basin
• Made up of creeks, brooks, ditches, and small rivulets
• Higher elevation (steepness)
• Higher velocity
• Higher energy
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Mid Basin
• Made up of larger stream bed channels that meander and form oxbows
• Increased wetlands and marshes
• Larger volume of water
and discharge
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Lower Basin
• Slower velocity but largest discharge
• Flatter terrain
• Larger floodplain
• Less meandering and increased sand bars with greater amount of sediment deposit
• River channel widens, but fewer oxbows
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Did you know there are three types of streams in watersheds?
Perennial Streams
Water flows in the stream at least 90 percent of the time in a well-defined channel.
Intermittent Streams
Flow generally occurs only during wet periods (50 percent of the time or less).
Ephemeral Streams
Flow generally occurs for a short time after precipitation events. The channel is usually not well defined.
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Stream ordering assigns a number to streams in a network to indicate their size and level of branching. The values are used to classify streams and identify their characteristics such as headwaters, tributaries, confluence, flow, accumulation, shapes of water channels, and topological distance from the outlet. For most methods, the smaller the stream the lower the number. This is very important to geographers, geologists, and hydrologists who use stream ordering to study, monitor, and measure waterways. Stream ordering methods include the Strahler, Shreve, Horton, Hack, and Topological. | |
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Strahler Stream Order Method
The most common stream order method, The Strahler Method, was proposed in 1952. This method assigns an order of one to the furthest upstream segments and increases the order by one at each confluence. The stream order only increases when streams of the same order intersect. For example, two first-order streams join to create a second-order stream.
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How To Determine Stream Order | |
To better understand the process, we have created an example using the Strahler Stream Order Method, to determine the stream order of the Knife River Sub-Basin in North Dakota. | | |
1= 1st order, 2= 2nd order, 3= 3rd order, 4= 4th order | |
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Putting It Together
Putting together what you have learned about watershed basins and stream ordering, the figure below demonstrates how watersheds (or drainage basins) collect water, and how flows in streams and river channels increase as they move downstream, again using the Knife River as an example.
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Starting at the upper reaches of the Knife River basin near Manning, North Dakota (ND), average annual flows at this location are 17 cubic feet per second (cfs). As we move downstream approximately 149 river miles to Hazen, ND, average annual flows are 170 cfs, or a 900% increase from Manning. This demonstrates how the watershed collects water from its upper to lower basins.
The Big Picture!
To look at this from an even larger geographic perspective, the Knife River flows into the Missouri River near Stanton, ND, the Missouri River flows into the Mississippi River at St. Louis, Missouri, and the Mississippi River flows into the Gulf of Mexico south of New Orleans, Louisiana – at an average discharge of just under 600,000 cfs!
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The Knife River at Manning, ND. | |
The Mississippi River at New Orleans, Louisiana. | |
In the next installment of The Watershed Thread, we will focus on water resource districts and state and federal agencies involved in managing water resources in watersheds. | | | | |
