A 17-year-old man died Thursday afternoon after being pulled from the American River near the Howe Avenue bridge, authorities said.

The man, identified by the Sacramento County Coroner's Office as Gian Louis Hernandez of Sacramento, reportedly was under water for at least 30 minutes.

Hernandez -- who was 13 days shy of his 18th birthday -- was pronounced dead at 4:36 p.m. at UC Davis Medical Center. Authorities are investigating the cause of death.

The incident took place about 3:45 p.m. The teenager was swimming near the Howe Avenue boat ramp when he apparently got caught in the river currents, said Sacramento Fire Department spokesman Capt. Jim Doucette.

A friend reported Hernandez looking tired before going underwater. Doucette said he was wearing shoes, which may have weighed him down.

Firefighters from the Sacramento Fire Department and Sacramento Metropolitan Fire District responded.

Family and friends are grieving, said Hernandez's girlfriend, Krystle Robertson, 17.

She described Hernandez as an outgoing, caring and selfless person.

She said Hernandez attended the former Highlands High School but was home-schooled later and completed his high school education this summer.

"He keeps me smiling when I'm mad or unhappy," Robertson said.

Authorities are advising people to stay out of the water or wear a life jacket if they must swim. The river can look deceivingly calm, Doucette said.

"They don't realize the dangers that are beneath the surface of the water," Sacramento Metropolitan Fire District spokesman Jeff Lynch said.

Currents just 6 inches below the surface are considerably faster than currents on the surface, Lynch said.

Even in the middle of the summer, the water temperature in the river ranges from the high 50s to the low 60s.

Swimmers also can become entangled in rocks, fallen trees or fishing lines, Lynch said.

Last year, 30 people drowned in Sacramento County. Ten of those drownings occurred in rivers, including the American and Sacramento. The remainder were in lakes, swimming pools, spas and hot tubs.

In the greater Sacramento region, 89 people drowned last year -- almost half of them in rivers, Lynch said.

Contrary to images portrayed in movies and television shows, victims don't usually splash around or make noise to draw attention when they are drowning.

"Drowning is a silent death," Lynch said.

Victims get tired, slip below the surface, inhale water and suffocate before someone realizes they have disappeared, Lynch said.

Authorities also warn against consumption of alcohol while swimming.

"Swimming and drinking is as dangerous as drinking and driving," Lynch said.

Authorities said it didn't appear that alcohol was involved in Thursday's incident.