For streams of intermittent or irregular flows, the structure and composition of riparian vegetation is usually related to water availability and flood disturbance. The hydrological recruitment niche, in which strong variability in water availability through flooding provides the foremost opportunity for germination and establishment of tree seedlings in riparian and floodplain areas. In the Pilbara region of north Western Australia large-scale iron ore mining activities such as the pumping of groundwater have large impacts on ephemeral streams and may affect the water balance by elevating groundwater levels and increasing water availability to riparian trees. We sought to quantify changes in the structure and composition of riparian and floodplain woodlands across a gradient of stream persistence and groundwater depth. Our results show that increased water availability in this dry semi-arid stream environment can create significant increases in the abundance, size and composition of the riparian tree community. However we found that the relationship between depth to groundwater and tree density and size across our study sites was not strong. This is most likely due to the important effects of landscape position and other edaphic factors such as soil nutrients and soil moisture variability. The hydrological recruitment niche has been somewhat disrupted at sites where stream flow is continuous due to mine water discharge upstream. Maintaining riparian communities in semi-arid regions requires specific management priorities, particularly in the face of anthropogenic disturbances such as mining operations. Therefore we need to better understand the relationship between the dynamics of groundwater and key riparian species. This requires a greater understanding of when, where and how riparian species are accessing groundwater on streams in semi-arid regions.