GLS100 Lab - Topography and Hydrology
Why do Peabody and Salem Flood?
Goal:
The goal of this lab is to learn how to read contour maps and to use them as an aid to
understanding landscape and environmental problems.
Objectives:

Determine elevation from contours

Define local relief

Approximate area using the UTM grid

Describe how lithology, structure, and glaciation influence landscape development

Locate and determine the orientation of streams and drainage basins

Determine the direction of runoff from topography

Explain the hydrologic equation and its significance

Approximate the groundwater table and direction of groundwater flow from ponds, lakes,
and swamps

Identify natural and anthropogenic factors that promote flooding

Discuss the effects of urbanization on stream flow

Identify areas of poor drainage

Evaluate and explain the causes of persistent flooding in Peabody Center and Canal
Street, Salem
Introduction to Topographic Maps
Map reading and interpretation is a required skill for all geologists. Field geologists must be able
to accurately locate data points and make interpretations. A well-trained geologist should be able
to read the landscape and identify significant features such as gravel-bearing deposits, faults,
landslides, drainage basins and flood plains from contour maps.
What is a Topographic map?
Topographic maps illustrate 3-D topography on a 2-D sheet of paper. Topography can be
represented either by shading or contouring elevation data. Shaded relief maps are easier to
visualize, but less detailed and harder to plot and retrieve data from than contour maps. In this lab
you will learn to read contour maps and make a number of interpretations addressing
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environment problem in the Salem area.
Terms
Contour: a line connecting points of equal elevation.
Contour interval: The elevation difference between contours
Map Scale: The length on the map that is equal to a given distance on the ground.
Elevation: height above mean sea level.
Relief: Difference between the highest and lowest elevation.
Height: relief of a feature. Measured from the base to the top.
Rules of contours:
1. By definition contours connect points of equal elevation.
2. Contour don't join or cross unless a vertical cliff or an overhang (very rare) is indicated.
3. All points enclosed within an unhachured contour are of a higher elevation.
4. All points enclosed within a hachured contour are of a lower elevation
5. Contours crossing streams V upstream (upslope)
6. Contours crossing ridges between streams V downstream (downslope).
7. The spacing of contours indicates relative slope.
Part I: Familiarize yourself with the map and answer the following questions:
1. Map properties:
a. Scale: State the relative fraction scale (______________) and the verbal scale in
miles/inch using the bar scale (______________mile/inch).
b. In a sentence state the meaning of the fractional scale:
c. Identify the contour interval:_______________ meters
2. Map area: The map is traverse by UTM grid lines spaced 1000 meters (1 km) apart.
a. Determine the east-west length ( __________ km) and north-south length (________ km) of
the map in kilometers.
b. Calculate the maps area in square kilometers? __________________km2
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c. Determine the approximate area that is land. ________________ km2
d. Determine the approximate percentage of land area that is urbanized? ______________ %
Explain how you determined this value:
3. Relief is the elevation difference between the highest and lowest elevations of an area.
a. Locate and state the elevation of the highest point of the land in the map area:
i. Describe location:_________________________________________________
ii. State Elevation (don’t forget the units):
_______________________
b. Locate and state the elevation of the lowest point of land in the map area:
i. Describe location: _______________________________________________
ii. State the elevation: __________________
c. Calculate the relief of the map area: _______________________________
Part II: Bedrock, structure, glaciation and topography
The map area is composed of igneous rock that intruded and cooled a few kilometers beneath
the surface, and was later brittley deformed by tectonic stresses. Deep weathering and erosion
over hundreds of millions of years brought this rock to the surface, allowing glaciers to etched and
scoured it over the last 150k years. The distinctive landscape of the North Shore it the
culmination of these events.
1. Describing the landscape: Circle the letter of the description that best describes the landscape
shown in the map:
a. flat land having a relief less than 10 meters, and dissected by a well-integrated system
of sinuous streams
b. hilly landscape with elongate north-east trending valleys and round symmetrical hills
c. poorly-drained mountainous terrain with a relief greater than 2000 meters
d. low-relief (<100m) landscape with irregular, asymmetrical hills, linear southeast and
southwest trending valleys of varying lengths, and a poorly defined drainage
system
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2. Signs of glacial erosion: Glacial ice sheets scoured the surface disrupting drainage systems,
excavating weak rock, and steepening the down-ice side of bedrock hills by glacial plucking.
Using your power of observation predict the direction of ice flow during the last glaciation, which
ended approximately 12,000 years ago. What aspect of the landscape supports your
hypothesis. (We will confirm or disprove this later when we go to Forest River Park.)
3. What features do you think control the location of valleys in the region?
4. In class we spent some time discussing the factors that promote mass wasting. Explain why
landslides are not a great threat to residents on the North Shore.
Part III: Hydrology
Geologists use the following equation to describe the partitioning of precipitation during a rainfall
event:
Hydrologic Equation:
P=PET+R+I
Where:
PET = water lost back to the atmosphere through evaporation and transpiration
R = runoff: water that flows over land and then directly into streams
I = water that infiltrates into the ground and groundwater table before entering
into streams
In a natural system, water flows slowly over land into rivers and streams, and seeps into the
groundwater table. Over time groundwater will slowly discharge into streams. The higher the
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percentage of runoff (R) during a storm the stronger the chance that flooding will occur. The
amount of runoff and its rate of flow is influenced by ground permeability, slope, and the type and
amount of vegetation--which intercepts rainfall and slows the flow of runoff. Percolation through
soil both slows the flow of water into streams and helps remove bacteria and contaminants. An
urbanized area is no longer natural but has been altered by the construction of houses, paved
roads and parking lots, removal of vegetation and installation of artificial cement channels and
underground s
ewer systems.
Figure 1. The effects of urbanization on stream flow.
1. Study figure 1. Describe and explain the effect urbanization has on stream flow.
2. How does urbanization influence water quality?
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3. Drainage and Groundwater Flow:
A drainage basin is an interconnected network of streams and tributaries that transport runoff and
groundwater discharge to the ocean. For example, the North River drainage basin includes at
least four streams that join near Peabody Center and flow into the North River. Within a
drainage basin some areas may be well drained and others poorly drained. Areas of poor
drainage, indicated by swamps and wetlands, typically occur in the lowlands where the water
table is near the surface. The water table will rise and fall in response to recharge from
precipitation, and in coastal areas will fluctuation in response tides and storm surges.
a. Locate and identify the major streams in the area.
Draining into Peabody: __________________________, ______________________________,
_________________________, _________________________ (these drain into the North River)
Draining into Salem: _______________________________________________
b. Are the stream network well-defined or appear interrupted? (circle one) Why?
c. Locating the water table: Lakes and ponds are formed where depressions are deep
enough to intersect the water table. Water flows down gradient, therefore the direction of
groundwater flow is determined by the slope of the water table.
i. State the elevation of the water table at Breeds Pond? ___________ meters
ii. State the elevation of the water table at Floating Bridge Pond near Buchanan bridge
_______meters
iii. From these and data obtained from the elevations of other ponds in the area state the
general direction of groundwater flow in the southwestern quarter of the map area.
___________
d. What is the approximate elevation of the water table in and around Needam Corner in
Peabody. ______________ meters
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e. Wetlands indicate a shallow water table. Is the water table shallow or deep in this area?
_______________________ How will a shallow water table influence flooding?
f. Look at the topography and drainage in and around Peabody Center, where Foster Street
and Lowell Street intersect. From your observations offer an explanation as to why this area
is so prone to flooding during major storms.
g. Now look are the region along Canal Street by the Athletic Center and explain why
flooding is common here also.
h. There are numerous water tanks and tower in the map area. Are these located on hills or
in lowlands. (circle one)
Explain why the water towers are located where they are:
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Part IV Summary:
Neatly summarize and explain all factors that contribute to flooding in the Peabody and Salem
area. Post your summary on the class wiki.
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