How does the river work?

When the lumberjacks first arrive to the forest at the start of the season, they build a small and cozy cabin for them to sleep and stay warm during all season. They will spend most of the week there, except for one day off, when they can go home. They also build a cabin for the animals, usually oxen that will pull the carts or the logs along paths that are made by hand.
When everything is setup they start woodcutting. When a log is first cut, it can be reduced a little bit by getting rid of some unnecesary parts, so it weighs less. They have special tools made for manipulating the huge logs so a single person can move an entire log, even though they may weigh a few tons each. With the help of an ox, the log is carried to the river, where the fun starts. Some times there are also cable systems that transports logs like in a zip line. Other times there are carts that travel on rails where logs are carried (maybe we will make a map inspired by these systems in the future).
The logs are all stacked in a small lake or similar, and after twenty days of cutting they call a "flood". The first floodgate on the river is opened and all the accumulated logs are slowly released, a process that usually takes two hours. . All logs travel along the river for another two hours, time in which a few workers have to make sure that no log gets stuck in the way. With the help of a special tool, similar to a pike, they literally ride the logs and jump from one to the other to push and control the wood so it can keep going downstream. Some workers walk on the riverside with the logs or are stationed in key spots like on platforms specially made for this job on bridges. Along the river there are floodgates that help retain the water and control the flow of the wood so no near village gets flooded, like it already happened in the past.
After a long, long trip down the river, the logs arrive to the Sawmill, where they are further processed and distributed.

Mechanics

At the end of each round roll a d4. Multiply the result by 5. The log and anything on top of it will move that many feet downstream. Do that for every log and you got yourself a moving battlefield.