RUNOFF: RUNNING THROUGH THE WATERSHED
Runoff is a part of the water cycle. In this activity you will see how runoff creates streams and how these streams are organized into watersheds.
Instructional Method: Demonstration
Goal: To show how water flows on the surface as part of the Hydrologic Cycle.
Objectives: Students will be able to:
Setup: 15 minutes
Runoff results when precipitation flows over Earth's surface. Gravity causes water to flow downhill. When water accumulates on the surface, it is funneled into streams and rivers which usually end up in an ocean.
Different rivers flow into different oceans. This is because rivers that aren't connected to each other belong to different watersheds. A watershed is defined as a river valley or group of adjoining river valleys where all of the water flows in a single direction to a single point of output. Watersheds have an overall shape like that of a deciduous tree. Streams represent twigs, and rivers represent branches. Like the branches of a tree, all these smaller rivers, called tributaries, join together forming the central trunk or main river.
Mountains, ridges, or other high points form divides which separate one watershed from another. In North America the continental divide separates the watersheds (ex. Columbia and Colorado rivers) that flow into the Pacific Ocean from the watersheds (ex. Missouri and Arkansas rivers ) the flow into the Atlantic. The continental divide is the highest continuous line and does not necessarily include the highest peaks. Rain that falls on the west side of this divide flows to the Pacific. Rain that falls on the east side flows into the Atlantic.
Hydrologists measure the magnitude of watersheds in terms of "order" by adding up tributary segments when they meet at a confluence. You start at the top or headwaters of the watershed and make your way down the main trunk to the bottom or foot of the watershed. A single order river consists of one tributary. A second order stream is created where two first order tributaries come together. A third order stream is made up of two second order streams... and so on and so on. Only the confluence of rivers of equal order bump the combined river up to the next order of magnitude. Therefore the confluence of a 2nd order and a 3rd order tributary does not change the magnitude of the combined river; it would stay a 3rd order.
First order watersheds may be only a few miles long with an area of less than 10 square miles. 7th order watersheds can be thousands of miles long and encompass millions of square miles. All but the largest watersheds like the Mississippi river system exceed a 7th order magnitude.
Why does water flow down hill? Where does it end up eventually? From where does it come? Do you have any rivers near you? Where do they start? What order are they as they pass near your town? Where do they end? What order is the river when it reaches its final destination?
As a homework assignment, assign students a short research paper (and/or an in- class presentation) on a particular watershed. Have the students describe the location and environment of the watershed headwaters and the bottom of the watershed and where various stages of the water cycle would be occurring. Using a non-detailed map like one found in a road atlas, have them calculate order of magnitude. NOTE: you might wish to assign an interesting river or two like the Humbolt river in Nevada which never reaches the sea.
Included National Parks and other sites:
Utah Science Core:
1st Grade Standard 5 Objective 1,2
Last updated: February 24, 2015