## 2 Methods  
### Study Area  
The Tualatin and Yamhill basins drain a portion of the Willamette Valley's northwest corner and are 1858 and 2000 km\({}^{2}\), respectively (Figure 1). The area holds the three broad land typologies of western Oregon--developed lands, agriculture and natural vegetation dominated by upland forests. A significant portion of the Tualatin basin lies in the greater Portland metropolitan area. Washington and Yamhill counties, whose areas approximately correspond to the majority of the study area, have experienced rapid growth since 1980 (Table 1) [22], continuing a legacy of population growth in the Portland metropolitan area over the last century [23]. The city of Hillsboro, a west Portland suburb, has more than tripled in population between 1980 and 2010. Washington County is seeing higher population density increase than the average for all counties of the north Willamette Valley region (Figure 2) [24]. Despite higher density, urban land cover continues to grow in the study area as population increases (Table 2) [25,26].  
To understand the drivers of the area's land cover change, a discussion of the policy context is warranted, as institutional factors mediate people's response to economic opportunities [27]. In 1973, the Oregon legislature created an institutional framework for land-use planning applying state-wide goals informed by citizens and implemented through local governments [28]. This regulatory environment was meant to first protect Oregon farmlands and also encouraged livable urban communities with well-planned infrastructure. A primary tool towards this end was an urban growth boundary (UGB) for each incorporated city to reduce urban sprawl and encourage future compact development. These UGBs effectively create a land use dichotomy of urban lands within and the resource lands on the outside. In 2004, the passage of state ballot Measure 37 allowed a compensation claim or waiver of development restrictions for property acquired prior to the enactment of the legislation [29]. Land use planning advocates convinced voters to replace it with Measure 49 in 2007. It provides a more rigorous definition of compensation by limiting claims to three or less new dwellings on a parcel. In 2007, Metro, the Portland regional governing agency, proposed urban and rural reserve areas (URAs and RRAs) surrounding the current UGB to plan for growth in a manner compatible with state land use goals, which include targeting areas for future development that limit impacts on ecological systems [30]. These basins and the Tualatin in particular have also come under a high level of scrutiny  
\begin{table}
\begin{tabular}{c c c c c c c} \hline  & **Pop. 1980** & **Pop. 1990** & **Pop. 2000** & **Pop. 2010** & **Ann. Ave. Change** & **Total Change** \\ \hline \multicolumn{2}{c}{**County**} & & & & & \\ \hline Washington & 245,860 & 311,554 & 445,342 & 529,710 & 2.39\% & 115.4\% \\ Yamhill & 55,332 & 65,551 & 84,992 & 99,193 & 1.90\% & 79.3\% \\ \hline \multicolumn{2}{c}{**Large Cities**} & & & & & \\ \hline Beaverton & 31,962 & 53,307 & 76,129 & 89,803 & 3.39\% & 181.0\% \\ Hillsboro & 27,664 & 37,598 & 70,186 & 91,611 & 3.94\% & 223.2\% \\ McMinnville & 14,080 & 17,894 & 26,499 & 32,187 & 2.70\% & 128.6\% \\ Newberg & 10,394 & 13,086 & 18,064 & 22,068 & 2.46\% & 112.3\% \\ \hline \end{tabular}
\end{table}
Table 1: Populations by decade for two counties and select cities in the study area.  
for water quality issues [31, 32]. Both basins have stream reaches placed on the state's 303(d) list for impaired surface water bodies in accordance with the federal Clean Water Act. Total Maximum Daily Loads (TMDLs) are in place or in development for several water quality indicators [33]. LULC plays a major role in determining water quality indicator values and stream health within both urbanizing and agricultural catchments in the study area [34, 35].  
Figure 1: Study area, including the current urban growth boundaries around each municipality.