To ensure that logs drifted freely along the river, men called "log drivers" or "river pigs" were needed to guide the logs. The drivers typically divided into two groups. The more experienced and nimble men comprised the "jam" crew or "beat" crew. They watched the spots where logs were likely to jam, and when a jam started, tried to get to it quickly and dislodge the key logs before many logs stacked up. If they didn't, the river would keep piling on more logs, forming a partial dam which could raise the water level. So when the jam crew saw a jam start, they rushed to it and tried to break it up, using tools and possibly dynamite. This job required some understanding of physics, strong muscles, and extreme agility. The jam crew was a really dangerous occupation, with the drivers standing on the moving logs and running from one to another. Many drivers lost their lives by falling and being crushed by the logs. Each crew was accompanied by an experienced boss often selected for his fighting skills to control the strong and reckless men of his team. The overall drive was controlled by the "walking boss" who moved from place to place to coordinate the various teams to keep logs moving past problem spots. For log drives, the ideal river would have been straight and uniform, with sharp banks and a predictable flow of water. Wild rivers were not that, so men cut away the fallen trees that would snag logs, dynamited troublesome rocks, and built up the banks in places. To control the flow of water, they built "driving dams" on smaller streams, so they could release water to push the logs down when they wanted.