How far upriver do the tides have an impact?
This question from a viewer named Larry falls beyond my area of expertise, which is why I consulted experts at NOAA and the USGS. The impact of tides on a river depends on the distance inland from the mouth of a river (where it meets the ocean), the depth of the river, the shape of the channel and the duration and magnitude of a given cycle from high to low tide (and back again). Because of the variation in those factors from one river to the next, tides may influence river levels for just a mile to as much as several dozen miles. The Columbia river, for example, shows a great response to tidal influence. It's big and it's deep. Tidal exchanges of 10 feet can occur at Astoria...(change in river level) and even as far inland as Vancouver, exchanges of 4 feet can and do occur.