If you could look back a century or two, you would see farmers and entrepreneurs constructing the first of dams that now punctuate virtually every tributary of the Hudson River.

Many of these dams, which were built for hydropower and flood prevention, are now crippled. Even defunct, they have a profound influence on the river ecosystem, and there is a growing movement among environmentalists and regulators to remove dams in the Hudson River watershed.

"Walk any of these streams, and you find dams mostly in the middle of nowhere, associated with some manufacturing that is no longer there," said Dan Miller, the state Hudson River Estuary Program's habitat restoration coordinator. "Not to say all dams are bad, but there are many that don't have value."

The Hudson River Estuary Program started its effort this year in the Fishkill Creek. This summer, Jesse Sayles is cataloging existing man-made stream barriers — including dams, culverts and buried stretches of streams — with the goal of identifying streams that would benefit from having barriers removed.

Stream continuum harmed

The roughly 357 dams and other barriers on the 63 tributaries of the Hudson River estuary block what is called the "stream continuum."

Sections of streams become lake-like, changing the flow of nutrients, sediment, dissolved oxygen — even warmth.

Ideally, nutrients such as nitrogen, phosphorous and silica flow unimpeded down a river. Along this path, plants and animals take up the nutrients they need to survive, Miller said.

Prevented from normal flow, some nutrients collect behind dams. This excess of nutrients causes some blue-green algae to grow in great numbers called a bloom. The algae will use all the oxygen in the water, robbing it from other organisms and creating a "dead zone" devoid of life.

Likewise with sediments, dams slow the water so much that sediment settles behind dams, rather than mixing in the water and flowing downstream.

In some cases, it's beneficial to trap sediment and nutrients that would otherwise affect a wider area downstream. Dams contain contaminated sediments, eroded soil that can bury fish spawning habitat, and excess nutrients that would otherwise flow into the estuary and could cause a dead zone there.

"Sediment is a big question in the Hudson Valley, and the things that get held up in those sediments," Miller said.

Ultimately, though, several experts said, a free-flowing stream is preferable because these derelict dams will crumble in time without ongoing maintenance.

Dammed water also heats up in the summer. Spilling downstream, it is often hot enough to shock cold-water species such as trout. Dissolved oxygen, which is essential for fish and other aquatic life, is also decreased as it flows downstream over dam.

Besides altering chemistry, dams put a physical barrier in the way of fish. Ocean-dwelling migratory fish that spawn in streams, such as American shad and river herring, are affected, as are American eels, which spawn in the ocean but grow to adulthood in streams. All three species have been in steep decline, but it isn't known how important a factor dams are in that decline.

In some streams where the first barrier to fish is a dam, as opposed to a natural waterfall, fish are blocked from historic spawning grounds.

"Most of the decrease in habitat for migratory fish is due to dams and barriers," said George Schuler, a representative from the Nature Conservancy.

Resident fish affected

Dams affect even fish and other living things that don't migrate to oceans.

These things might not happen immediately. It takes time, often at least two years, for the original ecosystem to return after a century of acclimating to dammed conditions.

Dams create ponds and other wetlands that are lost if the dam is removed.

Some are concerned with the effect barrier removal will have on the wetlands that have formed behind many dams. If the dams are removed, these wetlands will soon dry up, destroying the habitat they had provided.

Miller calls the problem a "natural resource trade-off" — more sustainable over the long term, since floods and time will remove most dams anyway if they're not actively maintained. Once the natural state of flux is returned to the stream system, new wetlands will form to replace the old, he said.

Each dam and barrier has to be assessed individually to determine what is the best solution. According to Sayles, "Barrier mitigation and removal is just one aspect of watershed restoration and rehabilitation."

Emily Morris recently graduated from Vassar College in Poughkeepsie. She was an intern at the Poughkeepsie Journal.

3 ways to restore damaged waterways

There are three general ways to restore natural processes in a river that has barriers.

• The first way is to remove the dam completely.
• The second possibility that has been successful in Europe is to dig a new channel around the dam, said Dan Kwasnowski, an environmental specialist with the consulting firm Devine Tarbell & Associates.
• The third possibility is to build a fish passage device. These do not restore the river continuum, but help with fish migration. Fish ladders, fish elevators and rock ramps allow fish to move up the river, but not all of these are easy to navigate, depending on how athletic a particular fish species is.