Recently, I downloaded some watershed data from the NHD website. I am taking a class that involves performing Linear Referencing the collected water quality data on the main creek running though the watershed. The first day went well with the referencing. The second day, when I started again the creek flow lines appeared as usual but when I moved the map in any way the lines all disappeared. If I restart the map the flow lines appear again but then disappear whenever I work with the map at all. The routes that I created stay visible. The coordinate systems of all the layers are the same.
Any ideas on why the creek flow lines keep disappearing?