The Tualatin River watershed in northwest Oregon drains 712 square miles and ranges from the densely populated areas of southwest Portland, Hillsboro, Tigard and Beaverton to agricultural areas near Scholls, Gaston, Banks, Mountaindale and North Plains to the forests of Oregon’s Coast Range, Tualatin Mountains and Chehalem Mountains. Most of the fast-growing urban population — approximately 500,000 residents — resides on 20% of the watershed’s area. Agricultural uses take up 30% and 50% of the watershed is forest.
The Tualatin River originates in the Coast Mountains at approximately river mile (RM) 80. It flows generally from west to east. The character of the river changes dramatically as it flows from the headwaters to the mouth. Its character is set in part by the gradient of the river and by the composition of the bottom of the river. The river can be divided into four general reaches, the upper section is called the mountain reach, the next reach the meander reach, followed by the reservoir-like reach, and finally the riffle reach. The following diagram shows how the elevations change in the main stem Tualatin as well as its upper three major tributaries; Scoggins, Gales, and Dairy (West and East Forks)
The mountain reach of the river, from the headwaters to about RM 55.3 (Forest Grove area) is very steep and has a basalt rock base. There are several waterfalls in this area. It is fairly narrow at about 15 feet wide and drops in elevation at a rate of about 74 feet per mile. The terrain is very steep. The predominant land use is forestry.
The meander reach flows from RM 55.3 in Forest Grove to RM 33.3 near the junction of River Road and Farmington Road south of Hillsboro. The predominant land use in this area is agriculture. The average slope in this area is 1.3 feet per mile and it has a width of about 50 feet. This area has a sediment base. The sediment is up to 150 feet thick and was deposited by the Missoula or Bretz floods approximately 15,000 years ago at the end of the last ice age.
The next reach is called the reservoir reach. It extends from RM 33.3 to RM 3.4 and has an estimated slope of only 0.08 foot/mile. This area is much wider with an average width of approximately 150 feet. It has several deep pools. It takes a very long time for the flow to go through this area. This area also has a sediment base. This area has a mix of agriculture and urban uses. The low slope of this area is caused partially by the low slope of the land but also by a low head dam at RM 3.4. This dam was constructed to raise the height of the river to allow it to flow down Lake Oswego Canal (RM 6.7) into Lake Oswego. Although the concrete base is still in place, the flash boards have not been used since 2003 to raise the level of the Tualatin River.
The final, or riffle, reach of the river extends from RM 3.4 to the mouth where it empties into the left bank of the Willamette River (Willamette River mile 28.5) near the city of West Linn. This reach is very different from the reservoir reach. Here the river drops 10 feet /mile. It is characterized by small pools and riffles and has a basalt base. This area is predominately urban.
The Tualatin River has five major tributaries. From west to east they are Scoggins Creek, Gales Creek, Dairy Creek (West Fork, East Fork, and McKay), Rock Creek (Beaverton Creek), and Fanno Creek. These all drain from the northwest or north into the Tualatin River or the left bank of the Tualatin River. They originate from the Coast Range on the west or the Tualatin Mountains to the north.
|River Mile of
|.......West Fork Dairy Creek||25.6|
|.......East Fork Dairy Creek||22.9|
Additional information can be found about these tributaries in the Environmental Reports section of this website.