Few quantitative studies have been done on the hydrology of fens, bogs and mires, and consequently any predictions of the cumulative impacts of disturbances on their hydrologic functions is extremely difficult. Bogs and fens are, in a sense, 'hydrobiologic' systems, and any evaluation of cumulative impacts on them will have to consider the complicated interactions, barely understood, among the wetland hydrology, water chemistry, and biota, and place the effect of individual wetland impacts within the context of the cumulative impacts contributed to the watershed from other geomorphic areas and land uses. It is difficult to evaluate the potential cumulative impacts on wetland hydrology because geologic settings of wetlands are often complex and the methods used to measure wetland streamflow, groundwater flow, and evapotranspiration are inexact. Their very scale makes it difficult to quantify the hydrologic function accurately. The paper reviews current understanding of the hydrologic function of bogs, fens, and mires at different scales and in different physiographic settings, and presents hypotheses on potential cumulative impacts on the hydrologic function that might occur with multiple disturbances. (Copyright (c) 1988 Springer-Verlag New York Inc.)