The large variation in the ratio of uranium-234 to uranium-238 ( 234 U/ 238 U) in rivers is not well understood, but may provide information about past weathering and rainfall and is important because it controls seawater ( 234 U/ 238 U). Here, we demonstrate the importance of physical weathering and rainfall for ( 234 U/ 238 U), using rivers from South Island, New Zealand. These data allow interpretation of an existing speleothem ( 234 U/ 238 U) record and suggest that New Zealand glacier advance 13,000 years ago was influenced by increased rainfall rather than by Younger Dryas–like cooling. A model of seawater ( 234 U/ 238 U) during glacial cycles indicates that rejection of corals based on modern ( 234 U/ 238 U) ± <0.01 is not merited and may reject the highest quality ages.