Modesto firefighters battled a stubborn three-alarm brush fire Saturday that burned two to three acres along the Tuolumne River, near one of the city's waste-water treatment plants. The fire was reported at 4:40 p.m. and firefighters had it under control within 20 minutes, said Battalion Chief Terry Inderbitzen. But because of the heavy brush and vegetation, it took firefighters more than two hours to make sure the fire was out, he said. The fire started on the north side of the river, near Sutter Avenue. But wind carried embers to the south side of the river, sparking two spot fires, which firefighters put out. Inderbitzen said six engines and 24 firefighters responded to the blaze. He expected firefighters to be mopping up until 8:30 p.m. The fire's cause is under investigation.