The purposes of this article are (1) to develop the new routing algorithm with genetic algorithm (GAF) and (2) to evaluate the advantages of GAF by comparing with previous algorithm (e.g. flooding type algorithm; FTA) by use of DDM completion. DDM completion is a new index proposed in this article and it indicates whether obtained DDM reflect altitude information of original DEM correctly or not. In this article, the following results are clarified; DDM completions against the original DEM of the GAF are higher than the one of the ETA for the low relief landforms and fans. For the high relief landforms, like mountains, the differences of completions between both algorithms are not clear. The adoptions of weights for mutation which correspond to the slope angles are useful to determine the flow direction on the error points. These results can offer the solution to improve completions of DDM on low relief landforms and fans where the FTA can not calculate correctly.