The Great Falls of the Potomac may not have the instant name recognition of, say, Niagara Falls.

But each year, about 3 million people visit this series of rapids and cascades along the Potomac River, where the rushing water and varied topography make for a beautiful outdoor canvas.

"It's very picturesque and scenic, like Yosemite, the Grand Canyon or Yellowstone," says Bill Line, a spokesman for the National Park Service, which oversees some 388 parks across America. "Most people don't realize it, but [this area] is one of the top 25 visitation sites nationwide."

Despite its popularity, there's often confusion about just what constitutes the Great Falls of the Potomac, which spans hundreds of acres in Maryland, Virginia and Washington.

On the Maryland side, Great Falls is part of the nearly 20,000-acre Chesapeake and Ohio National Historical Park, home to the historic C&O Canal that winds from Georgetown to Cumberland.

The area is a haven for hiking (the woodsy Billygoat Trail is especially well-traveled), bird-watching, fishing, nature walks and more.

The falls' Virginia portion is situated inside the 800-acre Great Falls Park in the town of Great Falls. Also part of the national park system, this side also has miles of multiuse trails and terrain suitable for such activities as rock climbing, horseback riding and whitewater kayaking (which park officials say is at one's own risk because deaths have occurred).

No matter which side of the border one chooses, the falls bring to mind terms like "natural wonder" and "force of nature," fitting, given their evolution.

Geologists believe they were formed during the last Ice Age, when the sea level dropped, forcing the river to cut its valley. The metamorphic rocks in the river are thought to be at least 550 million years old.

"The falls are similar to Niagara Falls, but the underlying geology is very different along the Potomac, because these rocks are very resistant to erosion," says C&O park ranger Rodney Sauter.

Because they've eroded less evenly and more slowly, he explains, the falls don't have quite the dramatic drop or gushing appearance of their New York counterpart. Yet they're still striking, plummeting some 40 feet from top to bottom.

"It's more like a series of drops. The amount of flow on the Potomac River affects it," notes Sauter. "If there's a high flow, you can barely tell they're there. With an average flow you will see more exposed rock."

The falls can be viewed from several overlooks in Virginia and one in Maryland, the latter accessed by a series of bridges and boardwalks leading out to Olmsted Island. Named for Frederick Law Olmsted Jr., the landscape architect and preservationist whose famous father designed New York's Central Park, the small island is a bedrock terrace forest that supports rare, threatened and endangered plant species. One might also spy a heron, small lizard or wild goose here.

The unique ecosystem that exists on the island today is a far cry from the 1940s, for example, when picnics, campfires and other public use degraded the area.

In 1972, Hurricane Agnes led to the closure of Olmsted Island and this fostered ecological recovery. Two decades later in 1992, a decision was made to enact the current bridge system, which limits where people can go on the island.

"It's very fragile," says ranger Sauter. "But the public can enjoy it in a sustainable way."

The staunch protection of Great Falls' natural resources does not mean its staff is less than welcoming to visitors; in fact, quite the opposite is true.

There are five visitors centers along the canal, including the Great Falls Tavern Visitors Center (in Maryland), which dates to 1828. The former lockhouse, hotel and tavern is now a focal point for guests to pick up self-guided maps and other information to begin their experience.

Ranger-led programs include tours and history walks; some days there are demonstrations of the lift locks that enabled mule-drawn canal boats to climb uphill; and there are various special events.