Soon, I’ll be sharing a big story I’ve been working on, about the safety of the Adirondack Park’s most hazardous dams. The story focuses on hard-to-miss dams that have been closely monitored by state inspectors for decades. The dams likely pose the greatest public safety risk of any of the 500 Adirondack dams listed on the state’s dam inventory.

But many dams – or at least what remains of them – do not show up on any dam inventory, even as they fragment river ecosystems and threaten the migratory ability of numerous species.

These unmapped dams, known as “ghost dams,” certainly exist in the Adirondacks. A recent project led by The Nature Conservancy removed a small dam in Reber this summer to help extend important habitat for trout and salmon. The dam was not on the state’s inventory. Conservationists say there may be hundreds of such undocumented dams throughout the region.

An image from the study showing as dots unmapped dams in a sub-watershed of the lower Hudson River. The diamonds are dams listed on the state inventory.

A study of the lower Hudson River watershed (outside the Blue Line) published last year used remote sensing data to develop a method to estimate the location of undocumented dams. Focusing on Latinntown Creek and Foundry Brook sub-watersheds on either side of the Hudson near West Point, researchers discovered scores of untracked dams by analyzing water flow patterns and satellite images.

The researchers developed and tested a “machine-learning” model to predict the locations of unmapped dams. They projected that the state’s current dam inventory underestimated the true number of dams by between 80-94% in studied watersheds. They found that unmapped dams were distributed throughout the watershed and estimated an average dam density of one dam per 1,000 meters of stream.

“Many of the first anthropogenic barriers that an aquatic organism would encounter when migrating from the Hudson River estuary into either of the two study subbasins were unmapped dams,” according to the study.

Let’s hope these researchers find their way to the Adirondacks.

Photo at top: A small dam in Reber removed last year fragmented stream habitat even though it wasn’t registered on the state’s dam inventory, many hundreds like it may exist across the state. Photo by Zachary Matson.

This first appeared in Zach’s weekly “Water Line” newsletter. Click here to sign up.