The floodplain of Johnson Creek is a remnant of large glacial floods known as the Missoula Floods that deposited thick sediments between 13,000 and 15,000 years ago in the Columbia River Basin, including much of the Willamette Valley. Underlying the sediments are substantial thicknesses of basalt lavas from the Columbia River Basalt Group. The lava, exposed mainly in the uplands, has been folded and faulted to form a series of sub-basins, including the Johnson Creek watershed.[11]

The watershed is a roughly rectangular area of about 54 square miles (140 km2).[4] The topography of the watershed varies greatly from the high point of about 1,100 feet (335 m) above sea level[4] in the Boring Hills near the creek's source to the low point of 26 feet (8 m) where the creek meets the Willamette River.

c range generally between 1 and 25 percent. Mount Scott and Powell Butte rise to about 1,000 feet (305 m) and have slopes ranging from 10 to 30 percent. Gresham Butte and Hogan Butte, in Gresham, have the steepest slopes, including a few around 50 percent. The Boring Hills, of volcanic origin, rise more than 800 feet (244 m) higher than the terraces to the north and west. The Kelso Slope, a northwest-sloping dissected surface west of the canyon of the Sandy River, tilts down from about 1,000 feet (305 m) above sea level near Sandy to about 400 feet (122 m) above sea level east of Gresham.[3]

Formed by the ancestral Columbia and Willamette Rivers, the terraces north of Johnson Creek are generally underlain by permeable sand and gravel. Three isolated hills—Rocky Butte, Mount Tabor, and Kelly Butte—rise about 200 feet (61 m) to 400 feet (122 m) above the surrounding terraces. With the exception of Powell Butte, the terrain on the north side of Johnson Creek is less steep than on the south side of the creek, which includes Mount Scott and the Boring Lava Domes.[3]

The eastern half of the watershed is more open and rural than the urban western half, which consists largely of residential areas with pockets of commerce and industry. An estimated 175,000 people lived in the watershed as of 2006.[4]The floodplain of Johnson Creek is a remnant of large glacial floods known as the Missoula Floods that deposited thick sediments between 13,000 and 15,000 years ago in the Columbia River Basin, including much of the Willamette Valley. Underlying the sediments are substantial thicknesses of basalt lavas from the Columbia River Basalt Group. The lava, exposed mainly in the uplands, has been folded and faulted to form a series of sub-basins, including the Johnson Creek watershed.[11]

The watershed is a roughly rectangular area of about 54 square miles (140 km2).[4] The topography of the watershed varies greatly from the high point of about 1,100 feet (335 m) above sea level[4] in the Boring Hills near the creek's source to the low point of 26 feet (8 m) where the creek meets the Willamette River.

Slopes in the watershed range generally between 1 and 25 percent. Mount Scott and Powell Butte rise to about 1,000 feet (305 m) and have slopes ranging from 10 to 30 percent. Gresham Butte and Hogan Butte, in Gresham, have the steepest slopes, including a few around 50 percent. The Boring Hills, of volcanic origin, rise more than 800 feet (244 m) higher than the terraces to the north and west. The Kelso Slope, a northwest-sloping dissected surface west of the canyon of the Sandy River, tilts down from about 1,000 feet (305 m) above sea level near Sandy to about 400 feet (122 m) above sea level east of Gresham.[3]

Formed by the ancestral Columbia and Willamette Rivers, the terraces north of Johnson Creek are generally underlain by permeable sand and gravel. Three isolated hills—Rocky Butte, Mount Tabor, and Kelly Butte—rise about 200 feet (61 m) to 400 feet (122 m) above the surrounding terraces. With the exception of Powell Butte, the terrain on the north side of Johnson Creek is less steep than on the south side of the creek, which includes Mount Scott and the Boring Lava Domes.[3]

The eastern half of the watershed is more open and rural than the urban western half, which consists largely of residential areas with pockets of commerce and industry. An estimated 175,000 people lived in the watershed as of 2006.[4]news live
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