Select Cross, then Caulk. The chance of success will be low. Rest and wait for more favorable conditions. Continue to rest until the chance of success is 59%. Then choose Caulk Wagon and Cross the River. So far, this has worked for me 100% of time. However, the entire party will have nearly 0 morale. To lift morale, use the harmonica and/or change Rations to Filling to slowly raise it.