Complex sentence: The Rhone has been an important highway since the times of the Greeks and Romans, but it was a difficult navigation, suffering from fierce currents, shallows, floods in spring and early summer when the ice was melting and droughts in late summer.
Split into simpler sentences: The Rhone has been an important highway since the times of the Greeks and Romans. Navigation could be difficult however, as the river suffered from fierce currents, shallows, floods in spring and early summer when the ice was melting and droughts in late summer.