Sylvia Thompson, a science teacher at the Beauvoir elementary school at Washington National Cathedral, retired in June, saying she wanted to spend more time relishing the outdoors.

" 'I love to kayak, and I never have time to do that,' " Paula Carreiro, head of the school, recalled Thompson telling her. "I expressed sadness about her leaving."

Shortly after 10 a.m. yesterday after an intensive search by air and ground, authorities found Thompson's body in her red kayak, which was floating upside down in two to three feet of water in Piscataway Creek, a Potomac River tributary, in Prince George's County.

Her husband, Dodge Thompson, reported her missing Sunday.

The cause of death had not been determined last night. Authorities said they were investigating the possibility that it was medically related.

"We do not suspect foul play," said Sgt. Ken Turner, a spokesman for the Maryland Natural Resources Police. He said that Thompson, 53, was not wearing her life jacket and that she might have removed it temporarily. It was found inside the kayak.

News of the death caused sorrow at the school where she taught for 17 years, Carreiro said.

Students, she said, were "very shocked. They were sad. Some of them were philosophical. They remembered she loved the outdoors and at least she was outdoors when she died."

Staff members said Thompson's approach to teaching left an indelible mark among legions of students who passed through the school.

"She really made science come alive," said Cornelia Atkins, a third-grade teacher.

"She was one of those people who not only loved children, but she was brilliant, truly," Carreiro said. "She combined that love and intellect in her teaching. It was evident everyday."

For example, Carreiro said, Thompson took a trip to Mexico to see the migration of the monarch butterfly and then related the experience to the students through pictures and words.

She had a great love of the outdoors and a great affinity for outdoor activities, including biking and kayaking, colleagues said.

"She was extremely skilled at kayaking," Carreiro said. "She was writing a book on kayaking in the area and was gathering research for her book on this trip."

Thompson, a native of Massachusetts, lived in Northwest Washington. She is survived by her husband, a curator at the National Gallery of Art, and two adult children. One recently graduated from Harvard University, and the other attends that school, colleagues said.