Flowing waters in deserts vary from ephemeral rills that carry water only after irregular and episodic downpours to the lowland stretches of perennial rivers whose headwaters are fed by groundwater interflow, snowmelt, or monsoonal rains. Many deserts have uncoordinated (arheic) drainage patterns. Here, flow may depend as much on where in the desert rain fell as on the weak gradients of the poorly defined channels. In other desert areas, meandering endorheic channels end in internal basins that can contain water for long periods of time. For example, the Lake Eyre Basin is a large endorheic drainage system in Australia that fills irregularly in response to erratic incursions of moist tropical air from the north. Major floods can occur, associated with La Nina phases of the El Nino Southern Oscillation (Puckridge et al., 2000), triggering 'booms' in productivity of waterbirds, fish, and invertebrates (Kingsford et al., 1999; Timms, 1999; Chapters 2, 4, and 7, this volume).