Lower Gales Creek Enhancement Planning
Geomorphic Assessment • Technical Study

Table of Contents

1.Introduction
2. Setting
3. Channel Conditions
4. Existing Conditions
5. Salmonid Restoration
6. References

2. Setting

Gales Creek is a major tributary to the Tualatin River and is located in Washington County, Oregon. The watershed encompasses approximately 78 square miles of primarily state owned and private forest land on the eastern side of the Coast Range. The lower watershed consists of a broad alluvial valley that opens up to the Tualatin Valley near the City of Forest Grove. Agricultural and rural residential uses dominate the lower valley. The City of Forest Grove owns a large portion of the Clear Creek watershed, which is used as a municipal water supply.

The climate of the Gales Creek watershed is characterized by a wet season and a dry season. The wet season typically runs from October to May with the dry season running from June to September. Rainfall totals in the western end of the watershed average up to 115 inches per year, whereas lower elevation areas on the eastern end near Forest Grove only receive an average of 45 inches of rainfall each year. This discrepancy is due to a phenomenon known as a “rain shadow” where rainfall gets squeezed out of storm systems on the western flanks of the Coast Range with less rain available for the eastern flanks and Willamette Valley. Most precipitation in the watershed falls as rain, though snowfall can occur.

Rock types within the Gales Creek watershed consist solely of young geologic material, primarily derived from the Eocene and Oligocene ages of the Tertiary period (Figure 2). Volcanic and sedimentary materials underlie much of the watershed creating highly erodible conditions where slopes are steep (Figure 3). The study reach, located along a low gradient, wide valley bottom, consists primarily of recent alluvial deposits that appear to be fairly shallow in some areas. Where the channel has incised deeply into the valley bottom, bedrock outcrops have been exposed. Two types of bedrock outcrops were identified in the field and can be differentiated by their resistance to erosion. The softer material is a marine tuffaceous siltstone and sandstone (Tss) that appears susceptible to incision. The more resistant material is a Mafic intrusion consisting of a massive granophyric ferrogabbro. This material appears to resist incision, meaning erosion into a Mafic outcrop would likely result in longterm stability of the local channel profile or migration of the creek channel into less resistant material.

3. Channel Conditions