We present a new and faster internal memory method to compute the drainage network, that is, the flow direction and accumulation on terrains represented by raster elevation matrices. The main idea is to surround the terrain by water (as an island) and then to raise the outside water level step by step, with depressions filled when the water reaches their boundary. This process avoids the very time-consuming depression filling step used by most of the methods to compute flow routing, that is, the flow direction and accumulated flow. The execution time of our method is very fast, and linear in the terrain size. Tests have shown that our method can process huge terrains more than 100 times faster than other recent methods.