These are "Boom Piers" they are man made island used to divide the river for multiple paper company's use of the river to transport logs. They extend about 8 miles
“The way to get their product here was to put the wood in the water and float it down,” explains Paul ‘Poof’ Tardiff, a historian and columnist for the Berlin Daily Sun.
All that wood, however, had different owners. Big players like the Brown Company and International Paper needed to access what was theirs. So they came up with a plan.
“They had to divide the river cause they both had the right to use it,” says Kelly. “So they built these islands of rock out of hemlock framing, and divided the river.”
In the 1890s, these companies started building piers: rectangles of different sizes, but generally around 10 feet wide, made of hemlock timber. They were then filled in with rocks.
Constructing a Boom Pier, circa 1893. They were built on the frozen river, then sunk during the flaw.
CREDIT COLLECTIONS OF GREATER RUMFORD AREA HISTORICAL SOCIETY, COURTESY WWW.MAINEMEMORY.NET, ITEM #80701
Further reading The Boom Piers: How Berlin, N.H.'s Islands in the Stream Came To Be, By TODD BOOKMAN • AUG 16, 2017, nhpr.org