Deltas and rivers on Earth are iconic landforms often associated to interactions with vegetation. However, these landforms also occur on Mars, which had fluid water once upon a time. We are learning about the past conditions on Mars by studying the remains of rivers and deltas, but we also learn about effects of plants on the landforms by interplanetary comparison. I will show results of experiments with rivers and deltas at a small laboratory scale and of comparisons between these systems on Mars and Earth. There will be two surprises: a very common river pattern on Earth is hardly found on Mars, and a very common delta form on Mars is hardly found on our planet but paints a clear picture what happens when sea-level rises fast