A combined sewer is an older infrastructure design where both sanitary sewage (from toilets, sinks, and drains) and stormwater (from rain and snowmelt) flow through the same pipes to the treatment plant. During heavy rainfall, combined systems can become overwhelmed, causing untreated mixture of sewage and stormwater to overflow into rivers and lakes—called combined sewer overflows (CSOs). Modern systems separate sanitary and storm sewers to prevent this pollution. Many older Canadian cities still have combined sewers in their historic cores, facing enormous costs to separate systems or build storage tanks to capture overflows. Addressing combined sewers is a major infrastructure challenge.