Chelsey Russell had run 100 miles through the rugged Rocky Mountains and completed the Boston Marathon twice. She snowboarded and skied and biked. She was comfortable in the water.

A 35-year-old attorney and mother of two who sat for the bar exam just two days after giving birth, Russell was, by all accounts, definitively tough.

She also had a heart condition.

It was that fateful combination that propelled Russell to fight desperately to save the life of her 2-year-old son last week — and ultimately, according to family, why she couldn’t save herself.

It started with a splash and a scream, and then another splash.

Recently divorced, Russell had just purchased her “dream home” and was finishing a relaxing summer last week with a family vacation to picturesque Lake Powell, reported the Denver Post, a reservoir that straddles the border of Utah and Arizona with walls of red rock formations surrounding miles of smooth water.

At 8 miles per hour Tuesday, Russell’s family cruised along the lake in a houseboat. On board was her mother and brother. Two children played on deck.

Suddenly, one — her two-year-old son — was in the water.

Russell dived in and swam toward the boy, reaching him before he slipped below the surface. He was not wearing a life jacket, authorities said. Neither was she. Russell propped him on her chest.

But it would take at least five minutes for her family to reach them.

According to a statement from the San Juan County Sheriff’s Office, the houseboat puttered a “significant distance” beyond Russell and the toddler before those onboard could kill the engine. Russell’s brother, Cayman Hood, jumped in after them, but quickly realized they were too far away, reported the Denver Post.

Lake Powell, Utah. (Photo by Justin Sullivan/Getty Images)

He swam back to retrieve the small motorboat they were towing, authorities said, but the knots tethering it to the vessel were so tight, reported the Associated Press, Hood had to cut them with a knife.

When he finally reached Russell, his sister was unconscious.

Yet somehow she managed to keep her son above the water and breathing. Hood lifted the toddler from her chest. He was conscious and cried.

Russell was limp.

Hood lifted her into the small motorboat, where she was given CPR. Life-saving measures continued for 30 more minutes at Hall’s Crossing Marina, but Russell never regained consciousness, authorities said. She was pronounced dead.

“We’d just had the most incredible week, our little family,” her mother, Trisha Hood, told the Denver Post. “It is unfathomable how this happened.”

Even law enforcement seemed awestruck by Russell’s sacrificial act. 

“She was still, for whatever reason, able to keep the baby on her chest, whether conscious or unconscious,” San Juan County Sheriff Rick Eldredge told the Gephardt Daily.

“She was holding the baby out of the water the best that she could,” Eldredge told the AP.

At first, authorities speculated that Russell’s cause of death was drowning, but family members told the Denver Post she suffered from a rare cardiac arrhythmia and had struggled with heart disease as a child.

The age she died, 35, was the same age her father — a prominent area orthodontist — suffered his first heart attack, childhood friend Quinn Washington told the Denver Post. When he died in 2013, Russell was “devastated” and became “the glue that kept the family together,” Washington said.

“It’s been one hit after another,” she told the newspaper.

Russell’s heart health has always been a priority, family and friends said. She was an active volunteer in the American Heart Association of Denver and was a founding member of Young Hearts, a group for young people with heart disease.

“She was a lovely lady who embraced who she was and the challenges she had,” Sheila Kemper Dietrich, former executive director at the AHA of Denver told the Denver Post. “She truly lived a life that recognized she needed to live a healthy lifestyle.”

Her law firm, Welborn Sullivan Meck & Tooley, posted a tribute to Russell on its website homepage.

“Chelsey was an amazing mother, an exceptional legal talent, an extraordinary athlete, a loyal and generous friend, and left us all better for knowing her,” her law firm wrote in a tribute on its website. “She is sorely missed.”

At the firm, Russell focused on “mineral title examination, business transactions and regulatory matters before the Colorado Oil and Gas Conservation Commission,” according to her staff biography. She was born and raised in Denver, and got both her undergrad and law degrees from the University of Denver.

Russell was the one who walked away from meetings having made everyone inside feel genuinely known. She hand-wrote her thank you notes. Former Colorado state judge Jack Berryhill, who knew Russell when she interned for him, reread the one she wrote him years ago after he heard of her death, WMAZ reported. He called her death “absolutely tragic.”

“She was extremely disciplined and tenacious,” Amy Seneshen, a partner at Welborn Sullivan Meck & Tooley, told the Denver Post. “She was tremendously driven and had an incredible way with people, where she could walk into a room and leave 30 minutes later, knowing every single thing about a person’s life. I’ve never met anyone like her.”

Seneshen told ABC News she was a “superstar in every aspect of her life.”

“She was a better mom than any other mom I’ve ever known,” Seneshen told CBS Denver. “She would have done anything for her kids.”

Russell leaves behind her two children, ages two and five.

“She was the most incredibly beautiful soul and spirit,” Russell’s mother told the Denver Post. “I was so blessed to have her for 35 years.”