did you add the correct column that should be plotted on x?
also I missed another error in your example, one closing parenthesis is at the wrong position.
try this:
p <- ggplot(thesisdate,
aes(x = date , # ???
y = salinity, color=stream_location)) +
geom_point() +
facet_grid(rows = vars(water_system), cols = vars(session),
scales = "free") +
theme_classic()
if this doesn't work, try without the facet_grid() first.