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Microtopography is often observed and studied in plant communities where environmental conditions limit distribution, for example in desert and peatland ecosystems. Brackish marshes, relatively poorly studied despite their importance in many coastal areas, frequently display similar fine-scale variability. In these systems, limits to plant distribution derive from the combination of flooding and salinity fluctuations. We examined the relationship between spatial variability of vegetation and biogeochemical features of sediment in a brackish tidal ecosystem in 2 adjacent Spartina spp. marshes of differing hydrology and vegetation distribution. The first, a low-elevation, interior marsh, was frequently flooded and poorly drained, and it exhibited distinct hummock/hollow topography. The plant distribution was reflected in patchy sediment biogeochemical features at the same scale. The second marsh, on a well-drained, elevated streamside bank, contained homogeneous vegetation cover and relatively uniform sediment chemistry. The formation of the hummock/hollow topography in the interior marsh appeared to be controlled by the plants, as they maximized growth in a high-stress, variable environment. The plants favorably modified discrete patches of these environments to such a degree that the hummock sediment biogeochemistry was very similar to that of the higher-elevation homogeneous marsh. The microtopography of this interior brackish marsh strongly resembles that of other stress-impacted ecosystems.