Q: I work in an office overlooking the River Thames and am always amazed at the big difference in water level between high and low tides. I have read articles about fairs held on the frozen Thames in winter; and even seen pictures of them. Obviously, the ice was thick enough to support people, horses and stalls. Assuming the ice did not rise and fall with the tide and that the river did not freeze at the highest tide of the year, did the ice flood at every high tide, forcing the occupants to leave as high tide approached? And what supported the ice at low tide, when the still flowing river might have been several feet below the ice?
A: The Thames is now constrained by the north and south embankments which were built in the second half of the 19th century. Previously, the flood plain was much wider and the relative difference between the height of high and low tides was much less pronounced. When the Thames froze it was during exceptionally cold periods when the climate in the 16th and 17th centuries was undergoing what has since been termed 鈥淭he Little Ice Age鈥�.
Such frozen river ice can be very strong and, when solid up to the bank, will become self-supporting. On the Thames it was thick enough to withstand the weight of horses and carts. During this exceptionally cold weather, any additional flooding which could break through and cover the ice would freeze and add to its thickness and strength. This would permit a significant gap under the ice at low tide. Of course, once the weather got warmer in the spring, the structure of the ice would weaken and local instability could cause flooding of the ice.
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A: In the 17th century, the Thames had no embankments, and was much wider than it is now. Therefore, the same amount of tidal water flowing in raised the level less than it does today. In addition, the broad piers and small arches of the old London Bridge restricted the water flow severely, more or less acting as a weir, so that the water upstream of the bridge was kept at a high level, and was almost nontidal.