The watershed can be divided into two hydrologic areas with different infiltration rates.
The northern area, comprising about 40 percent of the watershed, consists of the Portland Terraces, and the southern consists of the Boring Hills and the Kelso Slope.
Most of the rain that falls on the northern area percolates into the ground, and most of the rain that falls on the southern area runs quickly into the creek or its tributaries.
Rain that falls on the north generally has a more gradual, longer-lasting effect, sustaining the base flow of the creek during dry periods.

About 40 percent of the tributaries that originally flowed over the surface of the watershed were piped or relocated during urban development, especially on the north side of the main stem.
Generally, the remaining free-flowing tributaries begin south of the main stem and run north; the major exception is Crystal Springs Creek, which begins as a groundwater discharge and flows south.
The other major tributaries are Hogan, Kelley, Butler, Sunshine, and Badger creeks.
Crystal Springs and Kelley creeks contribute more flow to the main stem than the other tributaries.

Fill at Foster Road and Southeast 111th Avenue usually prevents stormwater runoff from a 9-square-mile (23 km²) area of the watershed in the Lents and Powellhurst-Gilbert neighborhoods from flowing directly into the creek.
Instead, runoff is routed to sumps, where it percolates into the ground.
Normal drainage patterns have also been altered further downstream in the Sellwood, Eastmoreland, Westmoreland, and Woodstock neighborhoods, where runoff flows into the Portland sewer system instead of into the creek.
The watershed can be divided into two hydrologic areas with different infiltration rates. The northern area, comprising about 40 percent of the watershed, consists of the Portland Terraces, and the southern consists of the Boring Hills and the Kelso Slope. Most of the rain that falls on the northern area percolates into the ground, and most of the rain that falls on the southern area runs quickly into the creek or its tributaries. Rain that falls on the north generally has a more gradual, longer-lasting effect, sustaining the base flow of the creek during dry periods.
About 40 percent of the tributaries that originally flowed over the surface of the watershed were piped or relocated during urban development, especially on the north side of the main stem. Generally, the remaining free-flowing tributaries begin south of the main stem and run north; the major exception is Crystal Springs Creek, which begins as a groundwater discharge and flows south. The other major tributaries are Hogan, Kelley, Butler, Sunshine, and Badger creeks. Crystal Springs and Kelley creeks contribute more flow to the main stem than the other tributaries.
Fill at Foster Road and Southeast 111th Avenue usually prevents stormwater runoff from a 9-square-mile (23 km²) area of the watershed in the Lents and Powellhurst-Gilbert neighborhoods from flowing directly into the creek. Instead, runoff is routed to sumps, where it percolates into the ground. Normal drainage patterns have also been altered further downstream in the Sellwood, Eastmoreland, Westmoreland, and Woodstock neighborhoods, where runoff flows into the Portland sewer system instead of into the creek.
Johnson Creek (Willamette River tributary)