The water source

A narrow river runs through the center of my town, and it ends at the junction with a big river at the edge of the downtown.
One day last summer, I rode my bike along the river. After a two-hour ride from the junction, I was supposed to reach its source, but I couldn’t approach it because of a tuft of grass.
The other day, which was a cold winter day, I went back to the same place and easily found the water source, which was a small shallow pond in a forest. I felt liberated from the unfinished investigation.