In this simulation the initial large-scale relief is flat, so that fluvial incision of inter-crater plains is modest.  Notice that during the final stages of infilling of craters on basin floors incised valleys sometimes develop rapidly and extend onto the basin floor, and then disappear just as rapidly as the crater is infilled.  Notice also that valley systems are often short-lived, particularly those that arise due to divide breaching between adjacent basins.  Continuing impact cratering can sometimes bisect or obliterate valley systems.