The potential for soil erosion varies throughout the watershed. Erosion is not a big threat in the northwestern part of the watershed, where the land is flat and developed, or in the northeast, dominated by low-erosion soils. In the southeast, soils have a medium risk of erosion, and soils around Powell Butte and the Boring Lava Domes have "an extremely high erodibility factor and are sensitive to ground disturbance".[3]

The watershed's soils vary in their permeability and water-retaining capacity. South of the creek and at the eastern end of the watershed they consist mostly of clays with high runoff potential. Northern areas of the watershed are generally porous, with moderate to high permeability.[3]
[edit] Hydrology
Two stream channels join below land covered with trees and shrubs and protected by a stone wall. The wall, about six feet (two meters) high, extends across the right-hand stream channel, which plunges over the wall. Downstream, the combined channels are about 30 feet (9 meters) wide and turbulent.
Lower Johnson Creek rejoins itself after forming a small island near Southeast 42nd Avenue.

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.[12]

Of the estimated 30 creek systems that existed in the watershed before 19th-century settlement, fewer than a dozen remain above-ground or free-flowing.[4] Generally, the tributaries begin south of the main stem and flow north; the major exception is Crystal Springs Creek, which begins as a groundwater discharge and flows south over relatively impermeable alluvial deposits to Johnson Creek at Johnson Creek Park.[12] The other major tributaries are Veterans Creek, Kelley Creek, Butler Creek, Sunshine Creek, and Badger Creek. Crystal Springs Creek and Kelley Creek are the largest tributaries in terms of flow contribution.[3]

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.[12]news live
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