EVERYBODY knows how much fun topographic maps can be. Here are some images with information dealing with the interpretation of topographic maps. Just "click" on each thumbnail to get an enlarged view.
The maps are presented in the order in which they appear in the lab book. These images are thumbnails. To view the larger size image, just left mouse button "click" on it.
EAST BROWNSVILLE, TEXAS
This general view shows the Rio Grande River. This is an EXTENSIVE flood
plain with EXTENSIVE meanders. Many areas of the map show evidence of
being eroded by the river.
This is a classic example of an old age river. Extensive meanders and
flood plain, low gradient and of course, natural levees.
Fort Brown Resaca is a remnant of a meander bend. This body of water is a
meander cut off called an ox-bow lake. (Ox bows have this shape.)
The Rio Grande River would not make a good political boundary because it is not stable. The river is subject to course changes after every flood.
As mentioned, this is an area in the stage of old age stream evolution. As a result most of the terrain is of low relief. On this map the high spot is in town and the low spot is sea level in the ship yard. The total relief is 38 feet.
SOUTH PASS, LOUISIANA
Note that in this legend (bottom center of map) there is no mention of the
contour interval (contour interval = 0). In addition, the relief is very
low. In the legend it mentions that the entire relief of the area is less
than 5 feet.
Deltas consist of water saturated fine grained sand, silt and clay. After
deposition it is normal that this type of sediment undergo compaction.
This portion of the delta is slowly sinking below sea level.
By the way, the blue "contour lines" in the water are actually bathymetric lines and have an interval measured in fathoms (1 fathom = six feet).
PROMONTORY BUTTE, ARIZONA
Confluence of Dick Williams Creek with Tonto creek (general view).
Close up view of above map.
Confluence of Tonto Creek with Christopher Creek.
Close up view of above map.
GRADIENT = RISE/RUN To determine the gradient of a portion of a stream you must determine from the map two numbers: the RISE (difference in elevation at the starting point to the ending point) and the RUN (the distance the stream traverses across the surface of the Earth).
Difference in elevation is 5760' - 5120' = 640'
Distance over ground is 4.75 miles
GRADIENT = 640' / 4.75 miles = 134.7 ft/mile. This is a fairly steep gradient and is associated with areas that are in the youthful stage of stream erosion. In addition to the steep gradient, there are no flood plains (another clue to the youthful nature of the area).
The highest point on Promontory Butte is 7920 feet. It is USGS policy to delineate the highest point of a summit on topographic maps. In many cases there may be a (D) depicting a bench mark - a survey monument placed in the ground by the USGS survey team.
The gray shaded areas (like those seen in the top images of Promontory Butte) are Non National Forest System lands within the National Forest. Many are private property.
Around Promontory Butte the drainage is RADIAL - the streams drain away from the butte. Elsewhere on the map the drainage is dendritic.
MAMMOTH CAVE, KENTUCKY
This portion of the map shows the REJUVENATED Green River. Meanders are
obvious (mature or old age stream features) but it also has steep canyon walls
as shown by the closely spaced contour lines. Steep canyon walls depict a
youthful cycle of stream erosion. This combination of features produces a
rejuvenated stream. In this particular case the river has cut deeper
(incised meanders) due to a general lowering of the water table in the area.
This questions draws your attention to hachured contour lines. These
special contour lines are use only for closed depressions. Water flowing
down these slopes has no way of flowing out (at least not on the surface of the
ground. The bottom of Hunt's Sink is between 640' and 620'. If it
were 620' there would be another contour line indicating that elevation.
So the best estimated would be less than 640' but more than 620'. Water
flowing into this sink hole can not get out, but it may flow underground.
CUMBERLAND, MARYLAND / WEST VIRGINIA
This is an example of an area that has a TRELLIS pattern of drainage. It
is a superimposed drainage. The ridges were buried below other rocks and
when the stream eroded away these upper layers the tops of ridges were exposed
to the erosive agents of the stream. The main stream was able to cut
through the ridges over time and maintain its course. The smaller
tributary streams, however, did not have the erosive capacity of the main
stream. When the harder rocks were exposed by the main stream the
tributaries were diverted. The result is the trellis pattern that is
typically found in the folded mountains of the Appalachians.
This is the Narrows and shows how the main stream was able to cut deeply through
the hill after many years of erosion.
BRIGHT ANGLE, ARIZONA (a bit more work on this one)
Bright Angel Creek
Bright Angel Creek drainage pattern
LAKE WALES, FLORIDA
The contour interval of this map is 5 feet. This is an area of generally low relief. Every 5 contour line (index contour) is 25 feet.
Many of the lakes have had their surface elevation measured and are shown as a blue number in the water.
Lake Elevations: Lake Effie 117'; Lake Pierce 76'; Lake Wales 112'; Twin Lake ~ 119'; Lake Serena 116' and Lake Annie 119'. In essence, the lake level shows the groundwater level in that area. Ground water will flow to the lowest lake. In this area it is lake Pierce in the north east part of the map.
Note the numerous closed depressions on the map. Many have water in them and are small lakes. These area of Florida is underlain by limestone and has been subjected to solution by ground water. The many depressions are sinkholes. Lake Serena is a classic example.
Serena Lake
For additional information on topographic maps, check out these Power Point Presentations:
SUNNYSLOPE QUADRANGLE - THE BASICS OF TOPOGRAPHIC MAPS (12.1 Mb)
This PPS is supplemental to this lab. It deals with the basics of how to read a topographic map including information on the Public Land Survey system.
TOPOGRAPHIC MAPS PART 2 (20.6 Mb)
This PPS is the next step in the study of topographic maps. It deals with the interpretation of landforms as well as the drawing of a topographic profile.