In order to connect the locations temperature data with the Site names, you would need to have a column containing the site name in both places. In other words, you would append the temperature values below one another. This would result in the date being repeated and the temperature all being in one long column.
You may want to append these data in a preprocessing step or they the Flatten data functionality to transform the data in DataGraph.
Once you have the temperature data in that format, use a Map column to populate a column containing the River distance for each Site name.
Does this help?