According to the Oregon Resilience Plan (page 204), the situation in 2013 was that
If it were to occur today, a Cascadia subduction zone earthquake would result in catastrophic impacts to existing water and wastewater systems throughout western Oregon. The Oregon coast would most likely experience strong ground shaking for over three minutes. Facilities within the tsunami inundation zone would be extensively damaged; in many cases, these facilities would not be repairable. Facilities outside of the tsunami zone would be heavily damaged, with total loss of water and wastewater services for periods measured in months and, in some cases, years.
The Willamette Valley would experience moderate ground shaking. Well-engineered structures may perform well, but many older structures would likely fail, including treatment facilities, reservoirs, and pump stations. One of the major impacts to large population centers would be from liquefaction: extensive alluvial and fill deposits along rivers would lose strength, lose bearing capacity, and move towards riverbanks. Old cast iron water pipelines buried in the liquefied soil would snap, and modern pipelines constructed of ductile iron and PVC would likely pull apart at joints, resulting in a total loss of water pressure throughout communities. Large drainage structures along riverbanks in liquefiable areas would likely move, severing connecting piping and rendering the structures useless.
Further (pages 208-9),
Potable Water Supplies In the current state of readiness, water utilities would be unable to provide water from the existing distribution system. Communities would rely on emergency supplies for the first one to two weeks, depending on location and on the condition of transportation infrastructure. Some areas would have no water supplies during that time. Water for healthcare facilities such as hospitals would be severely restricted. Emergency water supplies would meet only subsistence needs (for example, direct consumption and very limited bathing). For the first one to two months, water would be delivered via tankers to smaller tanks and bladders distributed throughout the community. People would wait in line to fill their containers and then carry the water home. Some water would come from portable water treatment units provided by the military, equipment suppliers, and foreign countries; however, the quantity of water supplied from those resources would be small compared to demands.
Note the qualification in the above paragraph regarding the “condition of transportation infrastructure.” Thousands of bridges and overpasses could fail, blocking every road between settled areas, and turning Western Cascadia into a vast collection of small islands without a means of transporting water and other heavy materials or equipment from one island to the other.
Over time, we may be able to reduce the vulnerability of water delivery systems, wastewater systems, and the road system to the CSZ event. Meanwhile, it is incumbent on all able adults to provide an emergency supply of at least 14 gallons of water for each member of their family. In addition, those responsible for taking care of vulnerable populations in the quake aftermath should find a way to provide those 14 gallons to each person who may be sheltering in their facilities. Those facilities would include, for example, long-term care homes, student housing, apartment houses, and businesses where there may be staff members, customers or clients present during the quake who can’t get home afterwards.
The 14-gallon target mentioned above is the current official guideline; however, Cascadia Prepared recommends increasing it where feasible to 30 gallons, especially in areas some distance removed from urban centers.
It may take even longer to restore wastewater disposal service than water delivery service. In addition to establishing emergency water reserves, it will be important in terms of both personal hygiene and public health to have a plan for disposing of solid and liquid waste for an indefinite period.