Many of the concerns associated with hydropower are caused by dams and dam impoundments rather than the hydropower technology (the actual generating equipment) itself. So let’s talk about those general concerns and about the very site-specific concerns related to the 2016 design plans for reusing the historic hydropower system.
General Hydropower Concerns
The concerns associated with existing dams and dam impoundments are mainly dam safety, flooding and impoundment greenhouse gas emissions. But there is also a multitude of ecological problems like migratory obstruction of local and migratory species, increases in water temperature, impacts on water quality, and the altered stream habitat. The exact concerns/problems differ from dam to dam and, in many cases, are site-specific to the dam setting.
Dams significantly alter the stream and stream-corridor ecosystem, which is why some communities decide to remove historic dams to return to free-flowing streams. Society can only mitigate the negative impacts of dams to a certain extent. But where removal is not an option due to recreational use of the impoundment, high cost of removal, flood control value and destruction of wetland above the dam, adding a low-impact hydropower installation can add another benefit: renewable energy generation.
The concerns associated with hydropower generation are related to fish entrainment, the created bypass (ecosystem), aesthetics of the installation, and impacts on existing recreational uses. All of these can be addressed and mitigated during site selection and system design.
But let’s talk about the site specific concerns at the Lower Saw Kill Dam.
Lower Saw Kill specific concerns
Despite its limited height and compared to other dams, the Lower Saw Kill Dam is a good candidate for hydropower: no flooding and dam safety concerns, a largely-shaded impoundment in the midst of a forest, and the only migratory species of concerns are American eel; Bard and the NYSDEC have already installed an eel ladder at the dam.
The 2016 design proposal suggested creating a 620-feet long bypass reach, from the dam almost to the mouth of the Saw Kill, including the stream stretch below the lowest waterfall, an ecologically extremely valuable stretch, see Kiviat, Stevens and Schmidt, 2017:
Between the Lowest Waterfall and the Tidal Mouth. This part of the Saw Kill is extremely dynamic in the spring with many migratory fishes entering the area from the tidal Hudson River, primarily for spawning. In the summer, this area is dominated by a very dense population of American eel and small numbers of other fishes. We know little about the fish fauna in the late fall and winter.
The following migratory fishes have been documented from this area of the Saw Kill. The anadromous species is limited to alewife (Alosa pseudoharengus) which spawn in the area annually. In years of high abundance alewife are seen in the pool below the waterfall but the waterfall is a complete barrier to their upstream movement.
The catadromous American eel (Anguilla rostrata) enter the tidal portion of the Saw Kill as “glass eels” annually; estimates are between 5,000 – 10,000. The population density of American eel in this area is one of the highest recorded at 1.2-1.4 eels/m2. The waterfall is a partial barrier to upstream movement of this species.
Potamodromous or facultatively potamodromous species include: white sucker (Catostomus commersonii), white perch (Morone americana), yellow perch (Perca flavescens), smallmouth bass (Micropterus dolomieu), golden shiner (Notemigonus crysoleucas), and carp (Cyprinus carpio). These species all spawn regularly or irregularly in the non-tidal portion of this area.
During the summer, because of the high density of American eel and their predatory activities, there are few other fishes in the area. Seventeen species have been collected […, Figure below].Kiviat, Stevens and Schmidt, 2017: Biological Surveys of the Saw Kill from Route 9G to South Bay, Town of Red Hook, Dutchess County, New York.
Fishes other than migratory species documented from the Saw Kill in the area below the waterfall and downstream to the tidal mouth.
The main concern about the proposed micro-hydropower project at the Lower Saw Kill Dam was related to the impacts the system would have on the rich stream life by reducing the amount of water flowing through this valuable part of the stream, by potentially even lowering the overall water level downstream the lowest waterfall as well as the attractive nuisance for fishes created by the outlet – specifically migratory species like the American eel, which might try to migrate upstream, following the flow coming from the turbine. As a result, the environmental community around the Lower Saw Kill Dam investigated dam removal instead.
But can a dam-free micro-hydropower system generate electricity at the Lower Saw Kill without having the same impacts? Let’s have a look when we talk about Dam-Free Micro-Hydro Design Requirements.