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Identifying Watershed Boundaries on Topographic Maps, Lecture notes of Geography

The University of Saint Mary (USM)Geography

This presentation provides a step-by-step guide on how to identify and draw watershed boundaries on topographic maps. It covers the basics of topographic maps, the process of delineating watershed boundaries using contour lines, and how to identify high points on a map. Test your skills by attempting to delineate the watershed boundaries for Lynn Camp Run and Laurel Run provided in the presentation.

What you will learn

What is the significance of contour lines in watershed delineation?
How do you identify the mouth of a watershed?
What are the primary lines on a topographic map?

How do you identify high points on a topographic map?
What is a saddle in geography?

Typology: Lecture notes

2021/2022

Uploaded on 09/27/2022

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Introduction

This presentation explains and illustrates how to identify and draw

watershed boundaries on a topographic map. The process is

explained step by step in the next 16 pages. You should review this

presentation until you have a good understanding of the process and

then test your watershed delineation skills using the two additional

topographic maps provided in pages 18 and 19 of this presentation.

The correct delineations (i.e., the answers) for those two watersheds

are provided on the last two pages of this presentation (pages 20 and

21). Note that the delineation of the second test watershed is more

difficult than that of the first because the landscape is flatter, which

makes the features shown by the topographic lines less obvious.

Partial preview of the text

Download Identifying Watershed Boundaries on Topographic Maps and more Lecture notes Geography in PDF only on Docsity!

Introduction This presentation explains and illustrates how to identify and drawwatershed boundaries on a topographic map. The process isexplained step by step in the next 16 pages. You should review thispresentation until you have a good understanding of the process andthen test your watershed delineation skills using the two additionaltopographic maps provided in pages 18 and 19 of this presentation.The correct delineations (i.e., the answers) for those two watershedsare provided on the last two pages of this presentation (pages 20 and21). Note that the delineation of the second test watershed is moredifficult than that of the first because the landscape is flatter, whichmakes the features shown by the topographic lines less obvious.

Drawing Watershed Boundaries When we talk about the hydrologic cycle and the water budget, wetypically talk about these things in terms of watersheds. So first youshould know what a watershed is. Watersheds

are topographically defined areas – something like bowls.

What we mean by that is that a watershed is separated fromneighboring watersheds by high points on the landscape. You can thinkof a watershed as an area that, if precipitation falling into it was notevaporated back to the atmosphere and did not infiltrate into the soil, allof the water in the watershed would run downhill to the same stream. Delineating watershed boundaries

. All watershed delineation means

is that you’re drawing lines on a map to identify a watershed’sboundaries. These are typically drawn on topographic maps usinginformation from contour lines. Contour lines are lines of equalelevation, so any point along a given contour line is the same elevation.

First you need to identify the stream whose watershedboundaries you are going to identify. So let’s say on this mapwe’re going to delineate the watershed boundary for Billy GoatRun. So look for blue lines and find the one on this map withthe name Billy Goat Run. It is in the lower left quadrant of thismap.

Next find the point on the stream that is the mouth of thewatershed. The mouth of the watershed is the furthestdownstream point of that stream, just where it meetsanother stream. By the way, the point where two streamsor rivers meet is called their

confluence

. Remember,

where contour lines intersect streams, they point upstream(the point of the V points upstream). Consequently, themouth of the watershed will be in the opposite direction ofwhere the contour line V’s point. So, the red circle marksthe mouth of the watershed.

x x

Go to the first contour line on the right side of the circle marking the confluence. The watershed boundary on that line wouldbe where the red X is located. Here’s why. If you put the point further to the right on that contour line, a line perpendicularto the contour line would point to the right of the confluence (see the green arrow) and water flowing that direction wouldn’tflow into Billy Goat Run. Instead, it would flow into the larger stream that Billy Goat Run flows into. If you put your first pointto the left of the X on that contour line, you can see the pink arrow indicates that water would move toward Bully Goat Run;which is what we want. HOWEVER, this point isn’t the highest point, because flow at the red X would still be in thewatershed and would flow approximately to the confluence. So this point is as far out on the contour line as you can goAND still have water flowing into the watershed. This X is the correct location for the start of the watershed boundarybecause the characteristics of that point make it the

highest

elevation on that contour line that would still allow water to drain

into Billy Goat Run.

Continue using this kind of thought process toidentify where the boundary makes most sense.Think about those “bulges” and the direction offlow at each contour line. Using this logic, thered X’s on the map, indicate where the nextseveral points on the contour lines should belocated to identify the watershed boundary.

x x

xxxx

xx xx

xxxx

So using that information you can identify wherethe next several points should be located.

x x

xxxx

xx xx

xxxx

x x

xxxx

xx xx

xxxx

x x

xxxx

Now you need to do some in-depth examination of the map tomake sure you’re still going to head in the correct direction.That is, if you continue looking for circles you may proceedfurther out a ridgeline, but it may be the wrong ridge. If youchoose the route shown by the red X’s on this map, and thendraw perpendicular lines anywhere on the circles surroundingthem, you’ll see that water would not flow downhill toward BillyGoat Run.

x x

xxxx

xx xx

xxxx

saddle

xxxx

You may get a bit confused at this point, becauseyou are now faced with a location where contourlines start coming together from opposite sides,but they don’t meet (i.e., the two blue contourlines). This type of landform is called a saddle,and saddles are very common on ridges. Theyjoin higher points on the ridge on either side ofthe saddle. As with the circles, the center of thesaddle is assumed to be the approximate highpoint on the ridge (the red X in the saddle).

Once you navigate through the saddle, you seeyou start to pick up the bulges again, so yousimply have to find the locations on these thatallow water to move toward Billy Goat Run. Asyou proceed marking the high points on themap, you’ll once again come to another highpoint within a circle (near the bottom of the pagewhere the last red X is shown).

x x

xxxx

xx xx

xxxx

As you proceed along the ridgeline toward the confluence

, pay close

attention to the direction of water flow.Remember you want the water to flowinto the watershed and not toovershoot the confluence point.

x x

xxxx

xx xx

xxxx

x x

x x x

x x

xxxx

Is this where you drew the last segment ofthe watershed boundary? If it is, you havecorrectly delineated the watershedboundary for Billy Goat Run watershed. Ifit’s not, compare your results to this map tofind where you went wrong. Remember,determining the flow direction will allow youto correctly delineate the watershedboundary.

x x

xxxx

xx xx

xxxx

x x

x x x

x x

xxxx

x x

xxxx

Identifying Watershed Boundaries on Topographic Maps, Lecture notes of Geography

Related documents

Partial preview of the text

Download Identifying Watershed Boundaries on Topographic Maps and more Lecture notes Geography in PDF only on Docsity!

Drawing Watershed Boundaries When we talk about the hydrologic cycle and the water budget, wetypically talk about these things in terms of watersheds. So first youshould know what a watershed is. Watersheds

are topographically defined areas – something like bowls.

. All watershed delineation means

is that you’re drawing lines on a map to identify a watershed’sboundaries. These are typically drawn on topographic maps usinginformation from contour lines. Contour lines are lines of equalelevation, so any point along a given contour line is the same elevation.

Next find the point on the stream that is the mouth of thewatershed. The mouth of the watershed is the furthestdownstream point of that stream, just where it meetsanother stream. By the way, the point where two streamsor rivers meet is called their

confluence

. Remember,

where contour lines intersect streams, they point upstream(the point of the V points upstream). Consequently, themouth of the watershed will be in the opposite direction ofwhere the contour line V’s point. So, the red circle marksthe mouth of the watershed.

highest

elevation on that contour line that would still allow water to drain

into Billy Goat Run.

So using that information you can identify wherethe next several points should be located.

saddle

As you proceed along the ridgeline toward the confluence

, pay close

attention to the direction of water flow.Remember you want the water to flowinto the watershed and not toovershoot the confluence point.

In this example, determine the watershedboundary for

Laurel Run

. The correct

watershed boundary delineation for eachexample is provided in the last 2 pages.