On the US east and west coast, charted depths are taken from what water level?

Charted depths on navigational charts are typically referenced to "Mean Lower Low Water" (MLLW). This standardizes water level measurements by averaging the lower low water levels over a significant period, specifically using the lowest tide that occurs during a tidal cycle. This reference point is crucial for ensuring consistency and accuracy in navigation, especially in areas that may experience varying tidal ranges.

Using MLLW as the reference point helps mariners avoid grounding and ensures that they account for the maximum submerged depths under various tidal conditions. This is particularly important along coastal waters where significant changes in depth can occur due to tidal influences.

The other reference points mentioned, such as mean high water levels or mean sea level, do not provide reliable measures for safe navigation since they can be affected by local conditions and weather, thereby varying significantly from place to place. Similarly, high tide levels also do not accurately reflect the depths that navigational safety is concerned with.