HARDWICK – Last week’s discussion looked at what the recent study found at Jackson Dam. This week, it’s worth stepping back and looking more closely at how rivers move sediment and why that matters.
When people think about a dam, they usually think about water.
But an important part of the story behind Jackson Dam is the sediment.
Rivers carry more than water downhill. They also carry sand, gravel, silt, ice and organic material eroded from hillsides and riverbanks across the watershed. Over time, that material moves downstream and settles out wherever the current slows.
Dams are one of the places where that slowing happens.
For more than a century, sediment from the Lamoille River and Alder Brook has been settling behind Jackson Dam. The recent study of the site found that large volumes of material have accumulated in the impoundment area, in some places far deeper than investigators initially expected.
That accumulation is not unusual. Around the world, reservoirs behind dams gradually fill with sediment. The process is slow enough that people often do not notice it happening.
But over decades, the landscape changes.
As sediment builds up behind a dam, the river channel upstream begins to behave differently. The river may become wider and split into multiple channels around forming islands. Flood flows can spread across newly formed deposits. What once looked like open water may slowly become mudflat, wetland or vegetated ground.
In other words, the river is gradually burying its old channel and rebuilding the valley floor at a higher elevation behind the dam.
That process has been underway for many years at Jackson Dam.
This matters because sediment affects how rivers behave during storms. A reservoir filled with sediment cannot store as much floodwater as it once could. Instead of acting like a deep basin, the area behind the dam becomes more like a broad, slow-moving reach of the river.
During large storms water moves through that reach differently than it did decades ago.
Sediment also shapes what happens downstream. Rivers depend on the movement of sand and gravel to maintain stable channels and healthy habitat. When that material is trapped behind a dam for long periods of time, downstream reaches can become more erosive as the river adjusts.
River scientists sometimes describe this as “hungry water.” The river below the dam carries a high amount of energy but less sediment than it would naturally, so it begins eroding the bed and banks in search of material to carry downstream.
A similar process can often be seen below undersized culverts acting like small dams, where scour holes or sharp drops form below the outlet.
For communities along the river, these changes are not always obvious from year to year. They become visible over longer stretches of time, especially when large storms reveal how the system now behaves differently.
SLR’s consultants spent considerable effort examining sediment when they conducted the Jackson Dam study because it plays such a central role in determining what options are possible in the future. Managing a structure like this is not simply about the dam, it involves understanding the material the river has left behind and concerns about dealing with it.
If future decisions involve changes to the dam, sediment will be one of the most important factors in how they are carried out. Stabilizing deposits, allowing vegetation to establish and managing how material moves downstream are all part of responsible river management.
Projects involving dams often take time because of that. Rivers operate on longer timelines than construction projects, and careful sequencing is often necessary to allow the landscape to adjust.
The story of Jackson Dam is therefore not just about a structure built in the early twentieth century. It is also about a river system that has continued to move sediment and reshape its valley for more than a hundred years.
Understanding that process is the first step toward deciding what should happen next.
Kristen Leahy serves as the zoning and floodplain administrator and the resilience and adaptation coordinator for the Town of Hardwick.
