This study reports a nonlinear relationship between urban floods, expressed as mean annual floods (MAF), and urbanization level, expressed as the percentage of urban areas within a watershed (PUAW). Analyses are mainly performed based on the 20-year gap-free daily mean streamflow and climate data from several hundred nested watersheds throughout the U.S. Mid-Atlantic region. This study discovers that increasing PUAW does not always lead to increasing MAF. Instead, a nonlinear threshold behavior exists. MAF first decreases with PUAW and then increases with PUAW after a certain threshold. When PUAW is smaller than a critical threshold value, land development percentage increases with drainage area; urbanization decreases with watershed size once PUAW goes beyond the threshold value. Repeating the same analysis over other regions suggests that a similar nonlinear behavior exists in water resources regions across the United States. The dominant mechanisms of the emergent patterns are being investigated by developing and testing different hypotheses. The scaling behavior may be used to support future urban flood mitigation and urban planning.