Stream channel enlargement occurs in response to the change in streamflow regimen accompanying urbanization. This empirical study relates the imputed increase in channel cross‐sectional area to detailed land use data and other information for 78 small watersheds near Philadelphia. Important differences between the effects of various types of impervious land use are observed: large channel enlargement effects are found for sewered streets and area of major impervious parcels such as parking lots, and much smaller effects are observed for unsewered streets and impervious area involving detached houses. Relatively low channel enlargement effects are attributed to all types of impervious development less than 4 years old and also to street and house area more than 30 years old. The influence of impervious development on channel size is found to be significantly related to topographic characteristics of the watershed, to the location of impervious development within the watershed, and to man‐made drainage alterations. Although the relative importance of these interactive factors proves difficult to establish, the most critical determinant of the amount of channel enlargement resulting from a given level of urbanization appears to be basin sl
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