On May 14, 1804, explorer William Clark wrote in his journal: "I set out at 4 O?Clock P.M. and proceeded on under a gentle breeze up the Missouri." What the members of the Lewis and Clark Expedition hoped to do was find an inland water passage across America from the Atlantic to the Pacific. In 1804, the only way to move a massive amount of cargo or people was by water. A northwest passage would make America?s westward expansion easy. As we know now, that passage didn?t exist. But in Lewis and Clark?s time the Missouri River flowed uninterrupted for well over 1,000 miles. Today, 200 years later, dams and power plants interrupt that free flow. In the second part of our ?Voyage of Discovery? series, Tim Wardner travels to the Garrison Dam, where modern times have changed the nature of the river and the surrounding environment.

Few things can harness the force of nature like a great dam. This is the Garrison Dam on the Missouri River in the midwestern state of North Dakota. Fifty years ago when the dam was built, few questioned the progress it represented. Now we realize that such structures can create unintended consequences.

?The dams were constructed for a purpose and it meets a lot of those purposes, but there?s always a cost,? explains Todd Lindquist, the man in charge of the Garrison Dam.

Before the dam, spring floods could devastate low-lying river towns. But the dam now controls the river.

?The dams have proven to be remarkably effective at controlling the flood events,? he says.

The dam brought irrigation water and hydroelectric power to the region. Tory Jackson grew up near the river. He is an environmental activist who is concerned about the management of the dam and river.

?When people talk about the Missouri River in North Dakota, a lot of them don?t realize there isn?t really much of a river left in North Dakota,? he says.

One reason to build the dam was to provide a navigation channel in downstream states. But the rise of the trucking industry meant fewer river barges.

?The amount of navigation they said would take place below Sioux City, Iowa, has never materialized,? Tory Jackson says.

?There is still a viable navigation industry downstream, but it never developed to the extent that they had anticipated when the dams were constructed,? adds Mr. Lindquist.

The dam reservoir is a benefit to boaters and fishermen, but putting barriers on a free-flowing river has threatened some species of fish and birds. Rob Holm works at the fish hatchery that supplies the Garrison Dam area.

?The habitat that was created with the reservoirs is completely different than these fish, the Pallid Sturgeon [need],? he says. ?They can?t adapt to that kind of environment.?

The Pallid Sturgeon is a migratory fish. It needs an open-flowing river to spawn. The Garrison Dam blocks the flow of the river and has lowered its temperature.

The controlled water release from the dam is relatively constant. Natural flow changes formerly exposed sandbars in the summer that waterfowl used for nesting.

The constant river flow tends to deepen the river in the center, a process called channelization. Bank erosion is another problem, especially if houses are near the riverbank. Lining the bank with rocks, called riprapping, can stop the erosion, but at a cost to the natural beauty of the river environment.

?There are some 250-year-old cottonwoods along the river that are unbelievably beautiful. They just don?t reproduce like they used to without the high water in the spring,? explains Tory Jackson.

Development along the river is also an issue. People love the beauty of the river, but their building houses next to it can diminish that beauty.

?We are not looking to stop all development along the river, we are just trying to do it with a little more common sense that doesn?t completely ruin the natural beauty of the river,? Tory Jackson adds.

?There?s no doubt that when you dam a river you change the environment, and there are many areas of the country that are dealing with this right now,? Mr. Lindquist says.

?We are not going to get rid of these dams anytime in my lifetime, certainly,? Tory Jackson says. ?So we are saying we only have this 87-mile stretch of river left, let?s do what we can to protect that.?

The control of nature has brought great advantages, but maybe we are just starting to see the real long-term costs of projects like North Dakota?s Garrison Dam.