1910+2013 230mw Holtwood Hydro Plant on Susquehanna River near Airville, PA

(Satellite)

I wonder if this is the largest fixed-weir dam in the USA. This photo was taken before another powerhouse was added in 2013. 

usfwsnortheast, Holtwood Dam, Susquehanna River Credit: USFWS

This shows the additional powerhouse that was built east of the original powerhouse.

LowImpactHydro
"The dam is an overflow-type structure that consists of a 2,368-foot-long by 55-foot-high, low hazard, concrete gravity dam. The top of the dam is raised 4.75 feet to an elevation of 169.75 feet via the use of wooden flashboards and inflatable rubber dam sections....The original powerhouse contains ten similarly sized vertical Francis turbines and two smaller vertical Francis turbine generator units, with a combined installed capacity of 171.4 MW. The new powerhouse contains two vertical Kaplan turbine generating units with a combined capacity of 80.6 MW....The project supplies a continuous 200-cfs minimum flow to Piney Channel to provide suitable habitat for resident fish species. Additionally, the project provides a continuous base flow of 800 cfs and a daily volumetric flow equivalent either to 98.7% of the inflow requirements of the downstream Conowingo Project or to the daily net inflow to Lake Aldred, whichever is less."

HydroReview
The $440m expansion was completed in 2013 and it included improved fish passage and a whitewater park. Construction began in 2010.

PAheritage
Construction began in 1905. "It was then the third longest dam in the world, built of solid concrete, 55 feet high....n 1912 the installation of the world’s first Kingsbury thrust bearing at Holtwood revolutionized the industry. The same thrust bearing remains in operation today and was designated as a Historic Mechanical Engineering Landmark by the American Society of Mechanical Engineers in 1987." It was built with two fishways. But they proved to be ineffective. Several fish passages have been tried since. An elevator lift was installed in 1997 and in 2014 modifications to the lift were completed.

ASME

ASME-brochure, p2

Satellite, LowImpactHydro has more information about the fish passages

"Shad and river herring are returning to their spawning on the Susquehanna at their lowest numbers since the 1980s, according to Exelon's news release. The population peaked in 2001, when hundreds of thousands of shad and river herring passed Exelon's Conowingo Dam, but that number has dwindled to 1,500 shad and 1,000 herring per year.Shad and river herring are returning to their spawning on the Susquehanna at their lowest numbers since the 1980s, according to Exelon's news release. The population peaked in 2001, when hundreds of thousands of shad and river herring passed Exelon's Conowingo Dam, but that number has dwindled to 1,500 shad and 1,000 herring per year....Per the new agreement, Exelon will invest millions of dollars to improve its dam's fish passageways and transport up to 100,000 American shad and 100,000 river herring to their spawning grounds through a trap-and-transport program each year, according to the release." [YorkDispatch, limited articles remaining]

UnchartedLancaster
230mw
"The oldest of the still operational lower Susquehanna dams is the Holtwood Dam, constructed between 1905 and 1910. Originally called McCalls Ferry Dam, it was later renamed in honor of two Pennsylvania Water & Power Company executives."
This photo is also in LancasterOnline

LowerSusquehannaRiverKeeper

Anthony Turner, Feb 2017

Tsvetan Nikolov, May 2020

The river can also get very low.

SueReno, her page also has photos of Lock #12 of the Susquehanna and Tidewater Canal.

SusquehannaRiverLands, this view is from the Face Rock Overlook.
"This observation point is located off Old Holtwood Road in Lancaster County.  It offers a view of the Holtwood dam, power plant, fish lift, and the Susquehanna River.  Interpretive signs and a map provide visitors with important information about the area."

reddit
Holtwood Dam, Pennsylvania. Person for scale.

I recognize the notch in the dam in the above photo. That photo would have been taken on the west side of the river.

Satellite

I think this is the 10" pipe that guarantees a minimum 200cfs flow to keep the fish alive. It also helps add disolved oxygen to the downstream water.

Satellite

Atkinson Construction
"This project features a [67'] tunnel from the Draft Tube Unit #1 of the Holtwood Powerhouse to the other side of the diversion wall. This created a controlled water flow to help the American Shad fish migrate upstream past the dam and also created water features for kayakers to enjoy."

WalshGroup has three photos concerning the 2013 construction of the additional powerhouse and improved support of fish migration.

1

2

3

LancasterOnline
$4m was spent on a whitewater park on the west side of the dam. It reused an old fish ladder and has a flow of 755cfs. The power company had to build this because their expansion project had removed "many of the riverbottom boulders that had created what some considered the best playboating features on the East Coast [I hit a paywall trying to access that link.]."