The win earns the Bearcats a berth in the MIAA Tournament, where they'll face No. 3 seed Missouri Western on Thursday at the Blue Valley Recreation Complex in Overland Park, Kan.
Northwest (20-28, 13-13) got to work quickly with three runs in the first inning, highlighted by Chantel Adams' two-run double off the center field wall.
Home runs by NSU's Saharra Henson and Ryan Martin tied it 3-3 in the fourth. The RiverHawks went ahead twice over the next two innings, but the Bearcats responded with a run to tie it each time.
Stephanie Mark delivered the game-winner, her fourth walk off hit of the year, with the bases loaded in the seventh as she lifted a ball that dropped into shallow right.
Paige Blythe (1-4) struck out two batters and gave up two runs off five hits in three relief innings.
The Bearcats started even hotter in Game 2, plating four runs in the first inning.
After NSU (26-28, 12-14) knotted it again in the third, Northwest pounded out another five in the bottom of the frame. Baylee Scarbrough and Taszia Roseberry each recorded three RBI while seven total Bearcats scored.
The teams traded runs until the RiverHawks held an 11-10 lead heading to the seventh. Mark launched a solo home run to send the game into extras.
NSU proceeded to plate three runners off Abbie Vitosh (7-12) in the ninth.
Adams' two-run homer pulled Northwest within a run, but Kayce Woodridge (14-10) kept the Bearcats from tying it again.
Vitosh tossed three innings in relief, giving up three runs on three hits while picking up a strikeout.
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