After the storm, the riverside became sludgy and slippery, making it hard to walk.

sludgy 是形容词，由名词 sludge（淤泥、泥浆）加上形容词后缀 -y 构成，表示"像淤泥一样的"或"充满淤泥的"。在这里形容雨后河边泥泞湿滑的状态。