Initial Publication Date: December 12, 2013

Stream Terraces:

Donald T. Rodbell, Union College


Intended Audience: Undergraduate course in geomorphology.

Location:

Stream terraces along a segment of the Plotterkill, a second order tributary of the Mohawk River that heads in the Helderberg Hills, just west of Union College (Schenectady, NY).

Summary:

This exercise involves field work surveying terraces along the Mohawk River and two of its tributary streams in eastern NY State. Lab write up requires students to present data graphically and to propose an hypothesis that explains the cause of episodic stream incision in the region.

Context:

This is a 3-week long project that is conducted as part of the fluvial geomorphology component of the course. We usually begin this project in late September when the leaves have begun to fall, so visibility in the deciduous forests is improved and students can see the flights of terraces without much difficulty.

We spend the first of three lab periods walking up a bedload-dominated stream bed that is a tributary stream to the Mohawk River. We notice the obvious flights of terraces along the stream. I ask the student to count them and walk up on them and think about how they came to be. We then think about how we might survey them, and document their presence. The students have a good idea what we're up to as we have stadia rods and abney levels. Then we go through the routine of measuring terrace heights using the modern stream as a datum. I then divide students into 3 teams and assign each team a different reach of the creek. In total we cover about 1 km., and teams are responsible for measuring the height of stream terraces above the modern stream and measuring with a tape measure their distance upstream from our starting point. On the second week we repeat this exercise on another stream, and then I compile all students data into an Excel spread sheet and ask them to do some simple calculations and plot longitudinal profiles of the modern streams, with terrace remnants plotted above the modern. They do this for both sides of each stream. They also plot a cross section of the stream valley at one point of their choosing to illustrate whether the terraces are paired or unpaired. On the final week, we tour the eastern Mohawk valley looking at much larger terraces that were produced by incision of the Mohawk, and we go to an outcrop of varves deposited in Glacial Lake Albany that crop out at the base of one of the terraces. The students thus get a sense of the large changes in base level that must have affected all the tributary streams in the region.

Students should know how to recognize stream terraces and how to conduct basic surveying using a stadia rod and abney level.

Goals:

The goals of this activity are to have students think about a classic problem in geomorphology--that is, the response of streams to climate change and base level change. Flights of stream terraces between 0.5 and 10 m above modern stream levels are ubiquitous in the northeastern NY (and much of New England, for that matter). The factors responsible for the step wise incision of these streams are numerous and include the draining of Glacial Lake Albany, post glacial isostatic rebound, and climate change via its effects on sediment yield.

The students can see the landforms clearly, and can understand conceptually how they could have formed, but the development of a hypothesis based on field data that they have collected requires them to consider such things as whether the terraces are paired or not, whether they are convergent or divergent with the modern stream, etc. Multiple hypotheses are certainly possible, and the students are asked to develop one and to suggest ways in which it can be tested.

Notes and Tips:

I have the student read the papers provided in the references section (below) to give them some background.

Assessment and Evaluation:

My evaluations of students work are based on their individual lab reports. Specifically, I evaluate whether they have considered multiple hypotheses for the origin of stream terraces, and whether they have done a good job of testing these hypotheses with the data they have, and whether they have proposed a reasonable means of testing the remaining hypotheses. The students who present their results to the class are also evaluated on the quality of their oral presentations.

Materials and Handouts:

 

 

References:

Bull, W.B., 1979. Threshold of critical power in streams. Geological Society of America v. 90, p. 453-464.

D. J. Merrits, K.R.V., and E. E. Wohl, 1994. Long river profiles, tectonism, and eustasy: A guide to interpreting fluvial terraces. Journal of Geophysical Research 99, 14031-14050.