Wetted perimeter is the length of wetted contact between the stream and its channel. It is measured perpendicularly to the direction of flow in a river. (Liu et al. 2006)
Liu, Suxia, Xingguo Mo, Jun Xia, Changming Liu, Zhonghui Lin, Baohui Men, and Lina Ji. 2006. “Estimating the Minimum In-Stream Flow Requirements via Wetted Perimeter Method Based on Curvature and Slope Techniques.” Journal of Geographical Sciences 16 (2): 242–50. https://doi.org/10.1007/s11442-006-0213-0.