The freezing point of water is 0 degrees Celsius (32 degrees Fahrenheit) at standard atmospheric pressure. However, the freezing point of water can be influenced by factors such as impurities, salinity, and the presence of other substances. The freezing point of the River Thames would depend on these factors as well as the prevailing environmental conditions.
Historically, the River Thames has frozen over during severe winter conditions. One notable period is known as the "Little Ice Age" between the 14th and 19th centuries when the river would frequently freeze over. However, due to various factors such as climate change and modifications to the river's flow, freezing over the entire length of the River Thames has become less common in recent times.
Generally, for the River Thames to freeze over completely, sustained cold temperatures below freezing would be necessary for an extended period. However, localized freezing, such as in shallow areas or near the banks, can occur even with relatively brief periods of freezing temperatures.
It's important to note that predicting specific conditions for the freezing of a river is complex and depends on a multitude of factors. Local weather conditions, water depth, flow rate, and other environmental factors can all influence the freezing process.