The pilot was landing a twin-engine turboprop airplane on a runway surrounded by snow berms. The pilot reported that as he approached the runway threshold, he reduced engine power to flight idle to descend, and the airplane's main landing gear collided with the snow berm at the approach end of the runway. A postflight inspection revealed substantial damage to the left wheel well bulkhead assembly. The pilot said that there were no preaccident mechanical anomalies with the airplane.