Urban runoff is categorized into two main sources of water, dry weather urban runoff and wet weather runoff. Dry weather urban runoff in the storm drain system occurs when there is no measurable precipitation. It originates from human activities, including car washing, landscape irrigation, street washing, dewatering during construction activities and natural groundwater seepage that discharges to the storm drains system. Dry weather urban runoff contains high levels of pollutants that enter the rivers, streams, estuaries and other marine waters in and around the City of Los Angeles. The City currently diverts approximately 6 MGD of dry weather urban runoff into the wastewater collection system for eventual treatment at the Hyperion Treatment Plant. In addition, there are permitted industrial discharges, which include discharges of treated reclamation plant effluent. During the dry season, the treatment plant effluent makes up much of the flow in the main channels of the storm drain. The dry weather season in the Los Angeles area typically runs from May through September.

Wet weather urban runoff occurs when there is precipitation that flows across the ground during and after a rain event; subsequently, draining into a storm drain system and discharging into the rivers, streams, estuaries and other marine waters in and around the City of Los Angeles. Wet weather urban runoff includes rainwater as well as most of the same flows generated during dry weather. The wet weather season in the Los Angeles area typically runs from October through April.