Ryan Forsyth won the race as the top-ranked Falcons grabbed seven of the top eight finishes for a third straight team title.
Carly Bunting won the girls race as her No. 8 Seahawks took the first three spots and the second team title in school history.
Severna Park's Forsyth won his race in a course-record time of 16 minutes, 6.12 seconds. Teammate Rhys Burgett was second (16:18.90), beating out South River's Brent Musselman, who broke up the Falcons' chance at a perfect score.
Severna Park finished with 18 points, beating out South River (76) and winning the county title for the sixth time in seven years.
"That was off everybody's radar," Forsyth said. "We weren't expecting to do that well. That was really awesome."
Dillon Cooley (fourth), Jack Kiely (fifth) and Clay Whitney (sixth) were the other Falcons who scored. Kevin Hines and JP Hussey finished seventh and eighth, respectively, , but weren't part of the team score; only the top five runners can earn points.
Severna Park coach Josh Alcombright was left to shake his head afterward.
"I always used to envision the best-case scenario," he said. "They just blew it out of the water today."
The South River girls took five of the top 13 spots to finish with 30 points. No. 7 Broadneck and Annapolis were next, with 59 points. A tiebreaker gave Broadneck second place.
Bunting won the race in 20:01.33, beating out teammate Reilly Wagner (20:12.76). South River's Caitlin Pierson took third (20:23.85).
"We knew what we were capable of today," Bunting said. "I turned around in the chute, and I saw all three of us. I wasn't surprised."
Claire Igoe (11th) and Katie Adair (13th) rounded out South River's top five.
"They just push each other," coach Hugh Harris said. "When you work hard in practice, it just translates to success in meets."
CARROLL COUNTY: Liberty's Sarah Rinehart won Thursday's title race, but Century placed three runners in the top 10 to win the team championship.
Doug Pickett of South Carroll won for the boys, but Liberty edged out Century by three points for the team title. Century took three of the top seven spots, but the Lions got five in the top 17.
BALTIMORE CITY: Darrius Salmond of Digital Harbor won Wednesday's race, leading his Rams to the city title at Herring Run Park. Salmond (17:15.91) beat out teammate Tyler Thornton (17:29.76).
Digital Harbor almost got a sweep, but Western edged out the Rams for the girls title. The Doves (59 points) topped the Rams by one point. Summer Gross of Dunbar won the race in 20:11.63. Western put three runners in the top 10, while Digital Harbor had two.