Imbrication: Learning stream science in Rochester Hollow

Posted on: October 20th, 2015 by Leslie_Zucker

A report on the recent Stream Walk through Rochester Hollow, by Watershed Educator Jen Bowman:

Rochester Hollow 9 (19)

A participant of the Rochester Hollow stream walk points to a pile of rocks along the stream edge and asks: “Why do the rocks do that? How do they get that way?” Along with the group, Allison Lent, Stream Assessment Coordinator for the Ashokan Watershed Stream Management Program, turns to look at the rock formation in question, and indeed, the rocks have created an interesting feature in the stream. They are sitting to the right side of the steam, and are the size of dinner platters. The rocks are stacked 4 deep, and sitting on an angle so that they overlap each other. Moss grows on the exposed end of the rocks that are above the water line trickling past them in the stream. The more we observe rocks in the stream, the more this overlapping pattern becomes apparent, and something tells us that there’s a story here as to why.

“That’s a great question, and keen observation.” says Allison. She then explains, “The rock layout and orientation that you see there is called “Imbrication”. She explains that imbrication is a term used to describe how sediment and rocks are moved and deposited by movement of water in the stream. Sediment and rocks moving along the stream channel are shifted by the force of water, and as the energy of the stream water fluctuates through changing slope of the stream bed, water volume, velocity, and channel impediments, those sediments and rocks are deposited along the stream, often into the slanted rock pile pattern we are observing.

AWSMP staff member Allison Lent talking about streams

AWSMP staff member Allison Lent talking about streams

As the group wanders up the trail, we continue to observe stream features and changes. We look at the glacial landscape of the valley and discuss its geologic history, and how the valley was shaped over time, especially in connection to the stream. Allison points out the terraced sides of the valley, and how those terraces were once active floodplains, but have long been abandoned due to the stream channel eroding downward, which is also called “downcutting”.

About half a mile along the stream, we observe several large hemlocks that have fallen across the stream. The trees fell in previous years, either by wind or stream bank erosion, and now provide many important benefits to the stream.

When trees and larger woody debris fall into streams, especially smaller headwater streams, the trees can change the shape and flow of the stream, impacting the entire ecosystem in significant ways.

Some of the benefits of trees in streams include improved habitat and diversity of aquatic and terrestrial organisms, trapping sediment, and preventing channel downcutting. Fallen trees also assist in maintaining floodplains, which help to dissipate energy of stream water velocity and flow.

Rochester Hollow stream with pool

Rochester Hollow stream with pool

As evidence of these benefits, we observe that the fallen trees in the stream created a place for the stream to form a small pool, which is vital habitat for fish and insects in the stream. The group also notices that the trees crossing the stream are preventing a great deal of rock, sediment and plant debris from washing downstream, slowing erosion and protecting water quality of the Rochester Hollow.

Continuing our walk up the trail, plant life around us transitions from stream vegetation to evergreen and deciduous upland forest ecosystems that were once used as farmland. Where the stream mingles with sediment, rocks and trees, a lush and diverse green carpet of flora thrives in the Hollow, showcasing the importance of the space now being part of a forest preserve landscape for many to enjoy.