The conservation of Helonias bullata L. (swamp pink), a federally listed species, is closely associated with the maintenance of appropriate site hydrology. Although changes to site hydrology have been implicated in the degradation and extirpation of some Helonias bullata occurrences, quantitative hydrologic data with which to assess potential hydrologic impacts to the species are lacking. We characterized site hydrology, substrate, topography, tree-canopy cover, and hydrologic regimes associated with Helonias bullata at two colonies located along small streams in the New Jersey Pinelands and assessed the potential impact of simulated water-level reductions on the species and its habitat. Over the two-year study period, surface-water levels at the two colonies fluctuated by 11.9 and 27.9 cm. Sites were characterized by muck substrate and variable topography with steep-sided hummocks in and along stream channels. Tree-canopy cover above the two colonies was 36% and 9% less than canopy cover in the adjacent forests. The pronounced difference in water levels associated with Helonias bullata plants compared with water levels associated with points where the plant was not present suggested that Helonias bullata was not uniformly distributed at the sites in relation to water table. Helonias bullata clusters, composed of groups of individual plants, were typically associated with the emergent portions of hummocks in and along the stream channels. Based on measurements at 958 clusters, the two-year median water level at the two sites was 7.9 and 10.9 cm below the base of the clusters. More than 90% of the total cluster area at both sites was associated with water levels between 5.0 (submerged) and 19.9 cm (exposed). The greatest total cluster area was associated with water levels between 5.0 and 9.9 cm, which may be the optimal water-level range for Helonias bullata. A relatively small simulated water-level drawdown of 15 cm exposed more than 30% of the cluster area at both sites to extreme hydrologic conditions, which we defined as the water level beyond which < 10% of the total existing Helonias bullata cover occurred at our sites (i.e., ≥ 20 cm below the base of the clusters). A larger simulated water-level reduction of 30 cm exposed all or nearly all of the cluster area to extreme conditions. Simulated impacts on habitat were less pronounced for smaller drawdowns because losses of suitable habitat (i.e., habitat that occurs within the 10^th and 90^th percentiles of measured water levels associated with Helonias bullata) were countered by dewatering of habitat that was previously submerged. The extent to which this dewatered habitat can compensate for losses in suitable habitat depends upon the potential for Helonias bullata to colonize the dewatered habitat. The hydrologic relationships described in this study may inform restoration efforts for this species and provide the basis for assessing potential impacts to Helonias bullata sites that are subjected to hydrologic variation.