Background:In the mid 18th century a dam was built at

0668249E 4923484N for the E.N. Colton shingle and saw mills along the Mill Brook in the town of Bolton, Vermont. During its stay a large wedge of coarse sand and small gravels were deposited by the Mill Brook on the upstream side of the dam. After the milldam let go around 1914, some 65 years later, the millpond that once existed gave way to an alluvial meandering channel. For the next 76 years, the channel had three different paths through the millpond sediments. The event steering the river to its present day channel was a large flooding event that occurred in 1990. In this event, there was a back up of water upstream due to a large woody debris dam, which caused the river to veer slightly west, cross cut a large meander and create a new incision through the existing sediments. There was likely a rapid sediment loss until the water finished cutting through the sediment at which time it met the coarser, original channel bar.

According to Stephen Wright’s work in the recent geologic past it is apparent that glacial processes have influenced the Mill Brook area. The glacial effects consist of varves, glacial till deposits and other correlations to major Vermont climate patterns.

Mill Brook valley have been affected by landslide activity in the past and currently. Scars and rotational failures can be viewed along the south wall of the valley.

Question:How much sediment has been incised from the moment the river jumped from its old path in 1990 to its present day course?

Methods:We started at the top of the sediment wedge and surveyed downstream calculating the difference in height between the top of the sediment wedge and bottom of the current river channel. At each survey point we also measured the width of the channel which provided us with the data necessary to calculated a volume of sediment incision since 1990.

incised

Beginning of sediment wedge. Woody debris dam to left.

Looking upstream near the middle of sediment wedge.

Remnants of old dam marking the end of sediment wedge.

Historic Photo

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Cross sectional diagram of Mill Brook to aid in river discharge value.

The Mill Brook River is located In West Bolton, Vt.

A dam similar to the dam in this picture was used at mill brook. Source: www.uvm.edu/perkins/landscape

Current photograph of Mill Brook area. Rachel Lomonaco and Alex Geller working in conjunction with Corey Coutu and Robert Zimmerman.

Designed by Alex Geller, and Rachel LomonacoUVM Geomorphology 2005

Representation of Historical image of site layout taken from Geomorphology 2005 Mill Brook Lab.

Volume

Cutbank

Flow

Results:

Stream Discharge = 0.77m3/sTotal Dissolved Load = 4.45 mg/LSediment Type= coarse sand to large bouldersChannel Type = graded from bedrock

to alluvial meandering back to bedrock.

Manning’s N = 0.054Sediment Incision Volume = 7764 m3

Calculations:Stream Discharge:Q = 0.572 m/s * 1.834 m2

Manning’s N: (0.097)0.5 ( 0.7707m2) (0.287m2/3)

(1.92 m3/sec)N=

Sediment Incision Volume:∑(Width*Length*Height)

Conclusion:Since the channel diversion

in 1990 we have calculated the sediment loss to be 7764 m3. We conclude that most of the sediment loss during the first flooding stages that have occurred to form the current channel.

Error can be accounted for by human inaccuracies in measurement due to poor surveying equipment. It was also raining during data collection which created a higher flow rate and dissolved load than normal.