Heavy rainfall can overwhelm such systems, sending excess stormwater, wastewater, and untreated sewage directly into other bodies of water.
In addition to flooding, heavy precipitation also increase the chances for mudslides on hills and even common unpaved walkways. Be careful.

NEVER attempt to walk into fast moving water. Move to higher ground. Do not drink the flood water and use it to wash utensils or dishes in. Use hand sanitizer and bottled water if you get stuck in a mud prone area.