Why couldn't Soviets trade on the Volga river during Stalingrad?

During the Battle of Stalingrad, the Volga River became a crucial supply route for Soviet forces. However, Soviet trade on the river was severely hindered due to intense fighting and the presence of German troops, who controlled areas along the riverbanks. The constant shelling and military operations made it dangerous and nearly impossible for Soviet vessels to navigate safely, disrupting normal trade activities and logistics. As a result, the Soviets relied heavily on the river for military supplies rather than commercial trade during this critical period.