i was surprised this weekend to discover how severely the river thames in london rises and falls with the tides... 5-7 meters!!
the tides occur because of the gravitational interactions between the earth, the moon, and the sun. the side of the earth facing the moon (closest to the moon) feels a slightly stronger force than the side of the earth farthest from the moon. the combined effect is to stretch the earth in the direction of the moon so that it is elongated slightly.
water feels the effect more than rock since it is more flexible. the earth rotates fully each day, so the bulged water parts that face towards and away from the moon, are changing as the earth rotates, and we witness the rise and fall of the earth's oceans (among other things). there are two tides each day as each part of the earth's surface rotates thru each bulged part.
the sun also plays this tidal game with the earth. the sun causes similar tides on earth, but they are half as strong as lunar tides since the moon is much much much closer! when the sun, earth, and moon are aligned, the effects of the tides from the sun and the moon add together and cause higher high tides and lower low tides. we call these spring tides and they occur twice a month (at the full moon and new moon phases).
the weakest tides occur when the moon is at a right angle from the sun, relative to the earth. in this case, the solar and lunar tides act in opposite directions and slightly cancel each other out. these are called neap tides and occur at the 1st and 3rd quarter phases of the moon. i dont know where the word "neap" came from.
i've seen tidal effects on oceans, but i dont remember previously witnessing such strong tides in a river! cool!
the shots of the tides i showed above were taken from the millenium bridge...
... which connects the tate modern museum (seen here on the other side of ruth)...
... and st. pauls cathedral (seen here from inside the tate modern!).