Original: On 22 October they found that the only way to pass the Murrumbidgee River, then in flood, was to convert one of the carts into a type of boat by passing a tarpaulin under it; the men, horses, and bullocks swam over, and everything was successfully got across.
Simplified: The Murrumbidgee River was in flood so they had to convert one of the carts into a kind of boat to get across.