Introduction.
Water flows downhill. From this simple fact follows a number of important ecological consequences. For one thing, flowing water is Earth's major sculptor, eroding deep and sometimes dramatic valleys, and chiseling at mountainsides. Flowing water also carries with it not only the water itself, but also added substances, such as nutrients and pollutants. As tributary streams join to form major rivers, the load of these nutrients and pollutants increases. Where the flow of the water is slowed, as it is in lakes, some of these substances may accumulate or settle out. Eventually, flowing water, and with it its accompanying load, finds its way to the sea.
Each body of water, whether it is a tiny rivulet or a major lake, draws its water from only a limited land surface. Water that falls on Aurora either evaporates directly from the land to the air, or it finds its way into Cayuga Lake. Water that falls on Moravia, a village a few miles east of
Aurora, finds its way, not into Cayuga Lake, but into Owasco Lake. Thus, somewhere in between there has to be a dividing line, a point where falling rain no longer flows west into Cayuga Lake, but east into Owasco Lake. This point, or rather this set of points along an irregular ridge line, is called a watershed, and no matter which way we travel from the shores of Cayuga Lake, we will eventually cross a similar divide. In other words, an irregular line or watershed surrounds Cayuga
Lake, running through points of high ground, on one side of which water flows downhill into
Cayuga Lake, but beyond which it flows elsewhere.
This watershed defines the Cayuga Lake drainage or catchment basin, and as such, demarcates an identifiable geographic unit suitable for study. We will refer to this portion of the local landscape as The Cayuga Basin. Precipitation falling on the basin will flow in streams and rivers into
Cayuga Lake. At the mouth of Cayuga Lake, at the lake's north end, the water flows out, joining the Seneca River on its way to Lake Ontario. Our immediate goal in this exercise is to delineate the Cayuga Basin on suitable maps and to determine the surface area of the Lake and of its catchment basin.
Reading Topographic Maps.
A map is a model of the landscape and can be made to emphasize any of a number of aspects of that environment. Most of us are familiar with road maps, which stress the location of roads, cities and towns. In this exercise you will look at a map that shows roads and other cultural features, but which also show physical features of the landscape, especially locations of streams and topographic relief ; you will use these features to determine the drainage patterns around
Cayuga Lake .
Look at the “Elmira” map of New York State. This map is drawn to the scale:
1:250,000. That means that 1 centimeter on the map is equal to 250,000 centimeters or 2.5 kilometers. There is also a ruler-like scale, which allows you to estimate map distances conveniently either in English
(miles) or in metric (kilometer) units. The lines of longitude are given along the top and bottom of the map, and the lines of latitude are given along the sides.
Notice that the orientation of the map is such that north is at the top, south at the bottom. This is "true" north and south. A compass points to "magnetic" north, which is slightly different.
Unlike most road maps, this map also shows a number of natural physical features of the landscape, most importantly, topographic relief. Topographic relief is shown here by contour lines. These are the light brown lines that join places that are the same elevation or distance above sea level. The elevation is written on the line, but you may have to search for it a little. The interval between lines is given at the bottom of the map, below the scale. Where those lines are

far apart (look, for example, at central Seneca County) the land is relatively flat, or has a gentle slope; where those lines are close together (see the area around Salmon Creek in northern
Thompkins County) the land is fairly steep.
Instructions:
I. Delineating the Cayuga Basin using a Topographical Map.
The main task for this exercise is the determination of the line corresponding to the watershed that makes up the Cayuga Basin. Fasten a large piece of acetate film over the “Elmira” map, centering Cayuga Lake under the sheet. Your acetate must cover the entire map from Seneca
Lake to the East.
Begin by tracing, with a colored marker, the shoreline of Cayuga Lake.
Next, (using the same color marker you used to trace Cayuga Lake) trace through any streams which empty into Cayuga Lake as these must, by definition, be inside the Cayuga Lake Basin.
You should at this point begin to get a better feeling for the extent of the basin.
As any stream which flows into another lake must be, also by definition, part of the drainage basin of that lake and not Cayuga Lake, trace (preferably in another color) those streams which border the Cayuga drainage basin, but which flow into either Seneca, Owasco, Skaneateles, or
Otisco Lakes.
Have you surrounded the Cayuga Basin with drainage basins? If not, what else might have a drainage basin? Trace these to the extent they help identify the border areas of the Cayuga
Basin.
You now have divided your map into two sets: first, those streams which flow into Cayuga
Lake, and, second, those which do not. The border of the Cayuga Basin must, therefore, lie somewhere between these two sets. Use the contour lines on the map to determine this border.
Remember that water flows downhill, so the divide between watersheds will always be at the high ground between them. Circumnavigate the basin, finding the highest points between streams of the respective basins and marking these points with a different colored pen. You will want to cross contour lines at right angles, because they run parallel to the slope.
Continue this process, following ridge lines and hill tops, and winding your way above stream valleys that form the drainage's borders, until you get back to your starting point. Finally, use your best skills to connect these points. When you have done so, you should see before you an irregular and somewhat elongated area: the Cayuga Lake Drainage Basin!
II. Estimating Areas by the “Cut & Weigh” Method.
What is the area of an irregular shape?
Finding the area of an irregular shape isn't too formidable a task if we do what we often do when confronted with such problems of estimation: don't measure the thing itself, but rather measure something that will be related in some predictable way to the thing we want to know. For example, instead of measuring the area of the Cayuga Basin itself, we will measure the area of the Basin as represented on a map.
But the question remains as to how the area of an irregular shape can be determined. Can you think of a property of the paper (or acetate) you are using to trace the Basin that is related in a predictable way to its area?
The weight of a piece of paper, regardless of its shape, is directly proportional to its area. We can thus use this relationship to determine areas of irregular shapes.

Using a piece of tracing paper or acetate, cut out several pieces of known areas. This process will be easier if you choose a convenient shape for your pieces, such as a square. You might start with a single piece of known area a little larger (maybe as much as twice as large) as the area you want to estimate and weigh that piece on an appropriately sensitive balance. You should repeat this process until you have a set of weights for 4 or so pieces. Record the weight and the area of each of the pieces. Now plot a graph of weight (the y-axis) vs. area (the x-axis). What is she shape of the graph?
Next, weigh the cutout of the feature you are interested in. Find that weight on the y-axis of your graph and follow that y value over until you reach the line of the graph. Find the x value that corresponds to that y value. What is the area of your cutout?
You should now have an estimate of the area of the paper or acetate cutouts of the basin and the lake. The last problem confronting you is to convert this map area to a real area in the landscape. To do that, you will need to refer to the linear scale of the map. It may be tempting simply to multiply your map area by 250,000 to find the landscape area, but let's think about it for a moment. What is the area of a square whose sides are 1 cm long? What is the area of a square whose sides are 10 cm long? Is the area of the second square 10 times the area of the first? What is the area of a square whose sides are 250,000 cm? Use this logic to convert your map areas into true landscape areas. Do these values seem reasonable to you? Convert your areas to other units. If you have calculated them km
2
, convert to mi
2
. If you have mi
2
, convert to km
2
, etc.
III. A Final Problem
So what? How might we want to use this information?
The Finger Lakes region of New York receives about 45 inches of rain a year. That means that if the Cayuga Basin were a very large, flat-bottomed, straight-sided pan, then in one year the depth of water covering the bottom of these pans would be 45 inches deep. What volume of water is this? (Remember to convert units appropriately.) How much would this water weigh?
Can you convert this weight and volume to units that make sense to you? The answers to these questions should convince you that it's a lot of water, and it has to go somewhere. If all the water that falls on the basin in a year were to drain into the lake, what would happen to the lake level?