an islet, rock in a river, embankment, to stop
坻 centers on a small landform in or near water: an islet, a rock in a river, or an embankment. The meaning 'to stop' likely extends from the idea of an obstacle that blocks flow.
坻 combines 土 (earth) with 氐, which likely contributes the sound. The character originally referred to an earthen mound or islet in a river.
The left side is earth 土, and the right side 氐 looks like a small mound with a flat top. Picture a tiny island of earth rising from a river, stopping the water's flow.
For チ, imagine a small islet where you can enjoy a cup of tea: tea -> チ, and the earth mound is your peaceful tea spot.