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.