What can change a 100-year floodplain to a 25- or 50-year floodplain?

The correct choice highlights the impact of reduced pervious surfaces due to upstream development on floodplain classifications. When land is developed, especially with impervious materials like concrete and asphalt, the natural ability of soil to absorb water is diminished. This leads to increased runoff during rainfall events, contributing to higher peak flow rates in water bodies. Consequently, this scenario raises the likelihood of flooding within a given area, which can effectively transform what was previously considered a 100-year floodplain into a 25- or 50-year floodplain due to increased flood frequency.

This transformation occurs because the metrics defining floodplain designations are heavily influenced by the watershed's hydrology. As runoff increases, flood events can become more common, thus necessitating changes in the frequency terms used to describe flood risk in that area. The modifications to land use and cover directly affect how water is managed within that environment, leading to higher flood risk classifications.

In contrast, options that mention increased soil absorption, higher tree growth, or more frequent drought conditions do not have the same direct effect on transforming floodplain classifications. Increased soil absorption would typically reduce runoff, while higher tree growth could similarly aid in water retention and absorption. More frequent drought conditions would generally lead to lower water levels and could reduce