Case Study: What Factors Influence the Way the Sudbury River Responds to Precipitation?

The Sudbury River Watershed

What happens to rain once it falls? Thinking about this question for a while and sharing some ideas with your teacher and classmates may help you later as you work on answering the question in the title of this Case Study. The Sudbury River flows along a northerly course just west of the densely-populated Boston, MA, metropolitan area. Within 30 miles of its headwaters, the river merges with a similarly-sized stream to form the Concord River. The United States Geological Service (USGS) stream monitoring gauge, which records the streamflow data used in this chapter, is located upstream from the point where the Sudbury River flows into the Concord River. Although the river's watershed is populated, it also includes a national wildlife sanctuary, and much of the land surrounding the river is wooded or in a natural state. In this chapter, you will investigate streamflow, precipitation, and the factors that influence the relationship between the two.

The small size of the watershed that is upstream of the gauge that measures streamflow—just over 100 square miles—makes it easy to study the streamflow-precipitation relationship. That's because in the case of the Sudbury River, the only storm systems that influence the river are the ones that occur in the immediate vicinity of the watershed itself. Imagine how much more challenging it must be to understand the flow of the Mississippi River at St. Louis, Missouri. Storms as far away as Minnesota or Montana will eventually influence the river's waterflow when it reaches St. Louis and other locations downstream.

There are different factors, however, that make the study of streamflow in New England difficult to understand. Sometimes the vegetation is active and growing, and pulling lots of moisture out of the soil. At other times the trees and shrubs are bare and dormant. Sometimes the ground is soft and absorbent, so water can seep in. At other times it is frozen solid. Sometimes the rain falls on snow-covered ground and sometimes it falls on a deep bed of newly-fallen leaves. These and many other conditions make a great difference in the way precipitation moves into a stream. The graph you create in this chapter will hold some surprises. As you study the data and have conversations with your classmates, you'll develop a better understanding of the relationship between precipitation and streamflow.