HCDN US daily runoff data
The HCDN dataset contains daily unoff data from the US only over the period 1874-1988. In general hydrologists are not keen to share their data, and with the exception of the US it is almost impossible to get hydrological data for use in the Climate Explorer.

This is the best I could find a few years ago, please contact me if you know of a more extensive or up-to-date dataset.