Where your water goes

Rain that falls on Portland ends up in the Pacific Ocean

1. Your sub-basin

   Willamette

2. then

   Columbia River

3. It arrives here

   Pacific Ocean

West, down some of the steepest drainage on the continent, and then straight into the Pacific. These are the salmon rivers: five species run up them from the sea to spawn and die, and the carcasses carry ocean nitrogen back into the forest.

Basin 170900 (USGS hydrologic unit)

Under your feet

Portland sits on Quaternary volcanic rock

About 3 million years old — from the ice ages and since — the most recent chapter.

All the kinds of rock, and how each one forms →

1. At the surface3 million years oldNeither way

   Volcanic rock

   Quaternary surficial deposits; Alluvial deposits · Quaternary

   What it is

   A catch-all for material erupted at the surface, from solid lava to airborne ash that settled and welded. What you get depends on how runny the magma was: thin magma flows, thick magma explodes.

   How it wears

   Ash rots to soil remarkably quickly — decades to centuries. Solid lava takes far longer.

   What it means above ground

   Fertile, mineral-rich ground in country that is usually dramatic. Volcanic ash holds water far better than its texture suggests, which is why ash soils grow well through dry summers.

2. Below, layer 23 million years oldNeither way

   Sedimentary rock

   Quaternary sedimentary · Quaternary

   What it is

   The family that covers about three quarters of the land surface, in a thin skin over everything else. Some geological maps say only this much, which still tells you it will be layered and may hold fossils.