Two everyday senses. As 'shallow lake' it appears mainly in place names like 白洋淀 (Baiyangdian). As'sediment / to settle' it lives in 沉淀, which also has a common figurative use: knowledge or experience 'settling' over time.
Left water radical, the side-form of 水. It anchors both senses: a body of water (shallow lake) and matter settling through water (sediment). Same family as 河 and 湖.
Right 定 supplies the sound — dìng giving diàn. Its 'to settle / become fixed' meaning also harmonizes with the 'sediment settling' sense, so the phonetic carries a faint semantic echo here.
