Complex sentence: The river is noted for its massive increases in flow volume and speed during rain which often causes the river itself bridges and the river itself to be impassable and even floods roads and train lines which in the these mountainous areas tend to be close to the river.
Split into simpler sentences: The river is noted for its quick and massive increases in both flow volume and speed during rain, which often cause bridges and the river itself to be impassable. This results in flooded roads and train lines which, in the these mountainous areas, tend to be close to the river.