The discharge regimes that occur in modern rivers vary chiefly with climate. Where there are large sand waves in a river, these show different responses to flow changes of different speeds and magnitudes. A rapid fall of discharge leaves bed forms stranded in the river, with flow around the front eroding the slip faces. With a slower rate of fall, smaller superimposed sand waves produce groups of convex-upward erosion surfaces within tabular cross-bedded sets. With a very slow fall and pronounced low-stage reworking, the sand waves are largely dissected.