Because her teammates depend on her, Margie Orrick roused herself from bed at precisely 4:48 a.m. She had set out her black tights and “Rock Creek Rowing” sweatshirt the night before, so she could be out of her Bethesda home by 5:06.

“We don’t have a lot of time to waste,’’ says Orrick, who is 58 and addicted to rowing on the Potomac River.

Her Prius pulls into the Thompson Boat Center in Georgetown at 5:28 a.m. She peels a banana for “fuel” as she steps into the chilly outdoors, lit only by a partly obscured April moon.

In the darkness, she can make out the silver hair of her teammates, who filled the parking lot in a span of six minutes. If even one didn’t show up, the configuration of the boat would be ruined. A few moments later, they gingerly place a 200-pound boat in trembling waters, ready to row faster than the sun can rise.

Orrick’s club has 50 members, men and women mostly in their 50s and 60s. Only a few have rowed longer than five or six years. The most common denominator among these athletes is that they once were curious parents who wondered whether crew team was as energizing as their teenage children said. So even as their kids moved on from rowing, the parents didn’t.

“It’s one of those things that if you stop, you know you won’t start again,’’ says Daniel W. O’Donoghue, 72, who began rowing in his early 60s. “And then you lose all the conditioning, and, at my age, it would be difficult to get it back.”

About three years ago, Orrick founded Rock Creek Rowing, one of a growing number of rowing clubs dotting the Potomac, Anacostia and Occoquan rivers. Club members practice four sunrises a week, over three seasons. They have two coaches and compete along the Eastern Seaboard in tournaments for people 30 and older, in a sport where there’s no room for flaking out.

The peer pressure is “the only thing that’s kept me exercising,” says Julia Wolf-Rodda, 52, of Bethesda, who thinks that rowing enables her to control her cholesterol levels. Most days, she carpools with Orrick, who picks her up at 5:07 a.m.

Orrick works as a technology consultant, a largely solitary existence. But that’s in the daylight. Now it is 5:48 a.m., and she is in the middle of an eight-woman boat, pulling out of the city and into nature.

Brent Keuch, their 29-year-old rowing coach, watches from a motorboat puttering alongside. During the rowers’ first strokes, the river is so dark that he has to measure progress by watching white oars slicing through the tiny waves.

Several hours later, he’ll be back on the water, coaching the crew team from Bishop Ireton High School in Alexandria. But he loves getting up early to spend time with “the ladies.” Sometimes, he gives them the same drills that he gives his high school students, “and they surprise me with how well they do.”

“I try to be mean sometimes, but I can’t,” Kuech says. “They are working so hard.”

The journey upriver glides past the Kennedy Center and the Watergate, national icons that are now just silhouettes in the pre-dawn dark. Then there is an occasional rumbling over the Key Bridge or the zoom of a car entering the George Washington Memorial Parkway.

But mostly, the soundtrack is of songbirds warbling around them. The sky lightens into watercolor purples and oranges, and a flock of ducks soars in V formation.

The city dissolves into a thicket of green where deer and geese reign. A heron flies to a rock, cranes its neck and keeps its eye on them.

From the distance, it all looks and sounds beautiful. Move closer and the artistry seems a lot more like hard labor.

“Come on, keep pushing ladies,’’ Keuch yells at them as his Flip cam rolls. “Push! Push!”

“Make sure to keep both hands on the oars,’’ he advises during a break in the water, near Fletcher’s Cove. “And control how deep the oar is going into the water.”

The women nod in agreement and vow to review the footage. And then, one asks:

“Did you see the heron?”

These are the benefits of rowing: The easy-on-the-joints, full-body workout. The chance to master teamwork. The concentration gained from mirroring the person in front of you.

But for Orrick, the entire thing can seem like a vessel for something more, something as big as nature. Two bald eagles are nesting near Fletcher’s Cove. If you look hard enough, there’s a sea turtle around there, too. The scent of honeysuckle is hard to ignore.

“It has been a healthy release for me,’’ Orrick said.

Returning to shore, the birds’ music is replaced by the honk of the car horn. The contours of the Kennedy Center and the Watergate become clear. Along the Mall, joggers run with earbuds coiling across their bodies like IVs.

When the Rock Creek Rowing group returns to the boathouse, the place is jammed with rowers from George Washington University and Sidwell Friends School, among others. They are half, even even a third, of Orrick’s age. About half are women, working together in a sport that Orrick couldn’t even fathom competing in when she was in high school.

“I missed out on Title IX,’’ Orrick says of the 1972 legislation that banned sex discrimination in education programs, including athletics, that received federal financial assistance. “When I was in college, sports bras hadn’t even been invented yet.”

Now, at 58, she has a team of her own.

It’s 7:30 a.m. when the boat is put away. Now Orrick has time to linger, discussing with her friends the need for more boating centers to alleviate crowds, a Saturday cleanup of the Potomac, the club’s competition schedule.

“It’s just wonderful to see the city wake up,” Orrick said. “It keeps my endorphins high for the rest of the day.”

If you have an idea for a story about the D.C. area at night, e-mail Robert Samuels at samuelsr@washpost.