Effects of Urbanization on the Hydrologic Regime of Johnson Creek, Portland, Oregon

The Johnson Creek watershed, Portland, Oregon, has undergone urbanization at an increasing rate. Excessive flooding has occurred in the lower portions of the watershed, resulting in an estimated 1.5 million dollars worth of flood losses annually. In attempts to mitigate the flooding damages millions of dollars have been spent to channelize the creek, and over 200 thousand dollars has been spent during the last 20 years on preliminary feasibility studies alone. The author saw a need to verify that there has been a change in the rainfall-runoff relationships in the upper Johnson Creek basin since 1941 and to quantify this change through the analysis of key hydrologic parameters. To accomplish this, extensive background research was conducted to gather as much extant information as possible about the climatic and physiographic characteristics that affect the basin hydrology. The watershed was found to have many distinct physiographic sub-areas comprised of different underlying geology, soils, geomorphic areas, slopes, vegetation patterns, and land use types that result in a complex interaction between the watershed input {precipitation) and the watershed output (runoff at the stream gage)