We show that a transitively reduced digraph has a confluent upward drawing if and only if its reachability relation has order dimension at most two. In this case, we construct a confluent upward drawing with \(O(n^2)\) features, in an \(O(n) \times O(n)\) grid in \(O(n^2)\) time. For the digraphs representing series-parallel partial orders we show how to construct a drawing with \(O(n)\) features in an \(O(n) \times O(n)\) grid in \(O(n)\) time from a series-parallel decomposition of the partial order. Our drawings are optimal in the number of confluent junctions they use.