Opportunities are rare to study streams undergoing primary succession. The Pumice Plain at Mount St Helens (USA) was formed by a massive pyroclastic deposit during the eruption in 1980. Several streams developed on the sterile ~15-km2 landscape shortly thereafter, and successional trajectories of instream benthic communities up to 35 years later varied substantially among streams. We asked whether differential development of hyporheic zones could explain any variation in benthic communities. During six months of the growing season (May-Oct) in 2019, we collected hydrological and geomorphological data across four Pumice Plain streams, and we thoroughly sampled benthic and hyporheic invertebrates in one representative reach per stream during July and August. Hyporheic zones were not well developed in any of the streams by 2019, and hyporheic communities were depauperate (range 2-23 taxa per stream) compared to benthic communities (34-63 taxa), with hyporheic assemblages significantly nested subsets of the benthos in each stream. However, streams with greatest abundance and diversity in the benthos had lowest abundance and diversity in the subsurface. Those streams occupied the western side of the Pumice Plain and had chemical and hydrological signals associated with crater springs in addition to strong upwelling throughout most of the sample reaches. Conversely, the eastern streams were likely sourced by glacier ice and snow present outside of the crater, and the study reaches showed signs of both aggradation and stronger downwelling than the western streams. Interestingly, the easternmost stream dries seasonally but had the greatest hyporheic diversity (prior to losing both surface and subsurface flow in Sept 2019). These contrasting hydrological and geomorphological settings from east to west across the Pumice Plain likely exert strong control on both benthic and hyporheic invertebrate communities at this still-early stage of primary succession.