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Scientific Investigations Report 2010–5065

Channel Change and Bed-Material Transport in the Lower Chetco River, Oregon

Chetco River

The Chetco River drains 914 km2 of southwestern Oregon and empties into the Pacific Ocean 5 km north of the California-Oregon border (fig. 1). Major tributaries are Tincup Creek (Rkm 54), South Fork Chetco River (Rkm 29), and North Fork Chetco River (Rkm 8.3; FPkm 7.6). In 1988, the Chetco River between Rkms 16 and 88 was designated as “Wild and Scenic” as part of the National Wild and Scenic River program. The eastern half of the drainage basin is within the Kalmiopsis Wilderness Area, established in 1964. At its entrance to the Pacific Ocean, the river separates the coastal communities of Brookings and Harbor. The drainage basin is entirely within Curry County, Oregon.

Geography and Geology

The drainage basin is steep and rugged. The highest point is Pearsoll Peak at 1,554 m, and the lowest elevation is sea level at the river mouth. The average basin slope is 0.42 m/m as measured from 10-m resolution digital elevation data. Drainage density, as measured from the 1:24,000 National Hydrologic Data set is 1.4 km/km2. The Chetco River is 88 km long, heading at an elevation of 540 m and descending to sea level at an average gradient of 0.006 m/m, but most elevation loss is in the upper part of the drainage basin, leaving the lowermost 38 km with a gradient of 0.0013 m/m.

The drainage basin is in the Klamath Mountains physiographic province, an amalgamation of several geologic terranes affixed to western North America during the late Mesozoic Era and early Tertiary Period in a progression of eastward dipping underthrusts. During accretion and subsequently, the rocks have been metamorphosed and intruded by igneous plutons, dikes, and sills, chiefly of Cretaceous and Tertiary age. The degree of metamorphism and igneous intrusive activity decreases westward, with most highly deformed metamorphic rock and intrusive igneous rocks forming the steeper and higher eastern part of the drainage basin, mainly upstream of Rkm 70. The western half of the basin is dominated by the Dothan Formation, which consists mainly of slightly metamorphosed greywacke sandstone and siltstone with minor amounts of volcanic rocks and chert (Ramp, 1975; Orr and others, 1992).

The steep slopes, high drainage density, and high gravel transport rates result from the combined effects of geologically recent uplift and erodible rock types. Analysis of uplifted 80–120 kiloannum (ka) shore platforms indicate late Quaternary uplift rates as high as 1 mm/yr (Kelsey and others, 1994), whereas geodetic and tidal observations suggest even higher historical rates of 2.5–3.5 mm/yr (Burgette and others, 2009). The rapid uplift has facilitated river incision and landsliding, especially in the upper drainage basin (Ramp, 1975).

The lower river valley, particularly along the lowermost 18 Rkm, has been strongly affected by the 130 m of sea-level rise after the culmination of the last maximum glacial period 18,000 years ago. Along the Oregon coast, rising sea levels have drowned river valleys incised during low stands of sea level, creating estuaries now extending inland from the coast. With the onset of sea-level rise, and especially during the last 2000 years of relatively stable sea level, these drowned river valleys have been filling with fluvial sediment (Komar, 1997, p. 30–32). For the Chetco River, the wide valley bottom of the lowermost 10 km is the result of this valley filling. Tidal effects extend 5 km inland, evidence that filling of the lower river valley has not yet matched Holocene sea level rise, and that the river has not yet attained a graded profile to the coast.

Hydrology

As described by early U.S. Army Corps of Engineers (1893, p. 3,431) navigation engineers, “Above the head of tide the [Chetco] river runs nearly dry in the summer, and is at all times but a small mountain stream, which becomes a torrent from the winter storms.” The combination of rugged physiography, high drainage density, and high rainfall associated with a Pacific marine climate results in high annual runoff values and flashy short-duration peak flows, but very low summer flows. Average annual rainfall in the drainage basin is about 2.4 m (Soil Conservation Service, 1979), ranging from about 2 m/yr at Brookings and increasing with elevation to nearly 4 m/yr in the basin headwaters (Maguire, 2001). Eighty percent of the precipitation falls during October through March, mostly resulting from 2- to 4-day Pacific frontal systems from the southwest.

Flow has been measured at the USGS streamflow gaging station (14400000; Chetco River near Brookings) at FPkm 15.24 since October 1, 1969. For water years (October 1–September 30) 1970 through 2008, mean annual flow has been 64 m3/s, equating to 0.75 m of runoff from the contributing area above the measurement station. Measured annual peak flows have ranged from 280 m3/s in 2001 to 2,169 m3/s in 1996; although, the 1964 peak is estimated to have been 2,420 m3/s. The mean annual peak flow is 1,085 m3/s (fig. 2).

To extend the record of peak flows to encompass the 1939–2008 analysis period, we estimated peak flows prior to 1970 on the basis of a linear regression between the Chetco River streamflow-gaging station (14400000) and the USGS streamflow-gaging station on the Smith River (11532500), near Crescent City in northern California that has been in operation since October 1931 (fig. 2A). Although the Smith River drainage basin, at 1,590 km2, is 74 percent larger than the Chetco River drainage basin, both are coastal drainage basins within the Klamath Mountains physiographic area subject to similar hydrological conditions.

The reconstructed peak flow history for the Chetco River shows a pattern of increasing annual peak flows during water years 1931–72, with typical values ranging from 700 m3/s in the 1930s to approximately 1,400 m3/s by 1970 (fig. 2A). Floods in the 1950s (particularly the 1955 peak flow event) appear similar in magnitude to the recent floods of 1971 and 2006, consistent with anecdotal records (Soil Conservation Service, 1979) that describe widespread flooding and damage associated with each of these events. Large floods with discharges exceeding 2,000 m3/s are much less common, and were recorded only in 1964 and 1996, although historical records indicate similar, if not larger, peak discharges during the large regional floods of 1861 and 1890 (Maguire, 2001). The estimated peak flows for 1931–69 do not show the extremely low values (less than 500 m3/s) such as those recorded in 1977 and 2001, although the regional drought in the 1930s coincides with generally lower annual peak flows for 1930-40.

Study Area

Our analysis focused on the lower 16 km of the Chetco River flood plain (fig. 1). The overall planform within the study area is that of a “wandering gravel-bed river” (Church, 1983) dominated by a single channel, but also having multichanneled reaches. The channel generally alternates position against opposite valley walls, forming deep scour pools where it flows against valley walls and shallow riffles where it crosses the valley floor between large gravel bars (Klingeman, 1993). The location and general shape of many of the expansive gravel bars (fig. 3) flanking the low flow channel are fixed by the control of valley geometry on high-stage flow hydraulics and consequent patterns of erosion and deposition. Within the study reach, the low flow channel as mapped in 2008 has an average slope of 0.0012 between FPkm 16 and FPkm 4.3, and a much flatter gradient in the tidally affected lower river and estuary. The channel has distinct pool-riffle morphology upstream of FPkm 4.5 (fig. 4).

Longitudinal patterns in gravel transport and channel change in the study area were characterized by dividing the area into five reaches of inferred similar transport on the basis of valley geomorphology, slope, and tributary locations (figs. 1 and 4, table 1). The Upper Reach (FPkm 13.2–16) extends from the upstream end of the study area to the Emily Creek confluence and is the most confined of all five reaches with an average flood-plain width of 213 m. The valley and channel widen slightly through the Emily Creek Reach (extending between the confluences of Emily Creek and Mill Creek, FPkm 13.2–10.6). The Mill Creek Reach (FPkm 10.6–7.6) encompasses the transition from the more stable upper reaches to the wider, more dynamic lower reaches, with flood-plain width increasing to 800 m as the Chetco River approaches its confluence with the North Fork of the Chetco River. The valley is widest along the lower part of the Mill Creek Reach and the North Fork Reach (FPkm 7.6–4.3), before narrowing and abruptly flattening as it enters the Estuary Reach, which corresponds to the tidally influenced zone from FPkm 4.3 (near the prominent local landmark, Tide Rock) to the mouth of the Chetco River (FPkm 0).

Land-Use and Landscape Disturbance in Chetco River Basin

Because of its rugged topography and remote location, the Chetco River basin was largely uninhabited until the early 20th century, and even today most of the drainage basin is publically owned and managed as forest lands and wilderness. Late in the 19th century, the U.S. Army Corps of Engineers (1893, p. 3,432) reported that “probably not over 100 people living in the whole Chetco Valley.” By the 1930s, individuals and lumber companies were logging on private lands along tributary valleys in the lower drainage basin (Chetco Watershed Council, 1995). Logging activity expanded to the upper basin during the peak harvest period of the 1950s–1960s, and then steadily decreased through the 1990s (John P. Williams, U.S. Department of Agriculture Forest Service, written commun., April 28, 2009). As of 2001, 97 percent of the Chetco River basin is managed as forest lands and wilderness by the U.S. Forest Service (USFS), Bureau of Land Management (BLM), and to a lesser extent, private timber companies (Maguire, 2001). More than one-half of the basin (521 km2), including much of the headwaters, is in the Kalmiopis Wilderness Area. Other land uses in the middle and lower basin include agriculture, rural residential development, and gravel quarries, which occupy 2 percent of the total basin area; whereas urban areas near the mouth of the Chetco River occupy only 1 percent of the basin (Maguire, 2001).

Forest Management and Fire

Although various natural and anthropogenic disturbances may influence channel conditions along the Chetco River, those disturbances likely to have the greatest effect in terms of sediment transport and channel planform along the study are watershed-scale disturbances such as floods, logging (and related) activities, forest fires; and activities within the study reach, including navigation improvements to the estuary, development and bank protection, and instream gravel mining. Logging and associated road building can increase peak flows (Wemple and others, 1996; Jones and Grant, 1996, 2001; Bowling and others, 2000) and the frequency of landslides (Kelsey and others, 1995), resulting in sedimentation along lower reaches of affected basins (Madej, 1995). Although data describing historical logging practices, road building, and resultant landscape change are sparse for the Chetco River, the peak logging in the 1950s and 1960s possibly affected sediment influx into the lower Chetco River.

In recent decades, two large regional fires burned parts of the upper Chetco River basin. The Biscuit Fire of summer 2002 was one of the largest historical forest fires in the Klamath Mountains, burning more than 57 percent of the Chetco River drainage basin with varying severity. In many places within the upper drainage basin, the Biscuit Fire overlapped with areas previously burned by the 1987 Silver Fire, although the Silver Fire burned only 10 percent of the basin (U.S. Forest Service, 2008). Possible long-term effects on Chetco River channel conditions resulting from the Biscuit Fire include enhanced runoff and erosion resulting from loss of vegetation (U.S. Forest Service and Bureau of Land Management, 2004), leading to downstream sedimentation.

Navigation Improvements

The Chetco River estuary is one of the smallest estuaries in Oregon, with a tidal prism extending approximately 5 km upstream of the Pacific Ocean, and its lateral extent constrained between steep valley walls (Ratti and Kraeg, 1979). Although the U.S. Army Corps of Engineers (1893, p. 3,431) originally declared that “the Chetco River estuary was unworthy of improvement” because of its small size and lack of regional commerce, expansion of the wood products industry and commercial fishing resulted in authorization of a series of navigational improvements as part of the 1945 River and Harbor Act (Slotta and Tang, 1976; Ratti and Kraeg, 1979). By 1959, a pair of jetties had been constructed at the mouth of the river, and an entrance channel was dredged through the bar that had historically blocked seasonal entrance to the estuary. Navigation and harbor improvements continued through the 1960s and 1970s, with the dredging of two boat basins in former tidelands areas and construction of a protective dike (Slotta and Tang, 1976; Ratti and Kraeg, 1979). The Port of Brookings completed these alterations by filling of a historical lagoon to reduce flooding and improve access to the moorages (Oregon Department of State Lands, 1972). Since the early 1960s, the U.S. Army Corps of Engineers dredged each year to maintain the entrance to the Chetco River channel, removing an average volume of 22,000 m3/yr (Judy Linton, U.S. Army Corps of Engineers, written commun., February 23, 2009; fig. 5). Only part of this dredged volume, however, is removed from the lowermost kilometer of the Chetco River, with the balance removed downstream of the jetties at the entrance to the channel. Additionally, it is uncertain how much of this dredged sediment, even that within the lowermost river, is derived from downstream river transport rather than marine transport into the lower Chetco River. For similar Oregon estuaries of the Yaquina and Alsea Rivers, most sand at the river mouth is of marine origin (Kulm and Byrne, 1966; Peterson and others, 1982). For the similarly sized (725 km2) Redwood Creek in northern California, Ricks (1995) showed that the sand in the estuary has a composition more similar to nearby Pacific beaches than that from the Redwood Creek drainage basin, indicating that a substantial part of the Redwood Creek estuary sand is from marine transport into the estuary.

Chetco River Gravel Mining

Sand and gravel has been mined for aggregate from bars flanking the low flow channel of the Chetco River flood plain for nearly a century. All this removal has been downstream of FPkm 16 and primarily has been in the estuary and near the confluence of the North Fork Chetco River at FPkm 7.5. Although historical records of removal volumes and practices are incomplete, accounts from long-time residents indicate that gravel extraction began in the early 1900s when gravel was removed by drag line from the estuary, and by the 1930s, several bars below FPkm 7.5 were being mined (T. Freeman, Freeman Rock Inc., written commun., 2009). Prior to 1967, no permit was required for instream gravel extraction in the State of Oregon, and on many rivers, it was common for aggregate to be removed from deep pits that extended well below the water line. Although anecdotal accounts (M. McCabe, Oregon Department of State Lands, oral commun., 2009; T. Freeman, Freeman Rock Inc., oral commun., 2009) indicate that several mining operators used such pits along the lower Chetco River, and aerial photographs from the 1930s to 1960s show possible water-filled pits on gravel bars downstream of FPkm 6, no records are available to better describe or quantify the volume of mining from this period. After the 1960s, pit extraction was gradually replaced with bar “scalping” or “skimming” techniques using scrapers or other heavy equipment to remove only the surface of the gravel bar, typically to an elevation close to the low-flow water level.

On the Chetco River, removal of instream gravel for aggregate probably peaked in the 1970s and 1980s, when at least 15 instream gravel mines were operating in the study area, and removal volumes were much higher than during recent years. Records listing removal volumes from a small number of mine operators show that average annual extraction for 1976–80 was about 140,000 m3/yr (Marquess and Associates, 1980), a rate three times greater than that for 1993–2008 (fig. 6). In 1994, the Chetco River was declared navigable (and hence publicly owned) by the State of Oregon, and royalty fees were assessed on instream gravel extraction. Largely in response to tighter permitting conditions and fees, only three companies have continued commercial gravel extraction on the Chetco River, and so the annual volume of gravel removal has decreased substantially. From 1995 through 2008, instream gravel was mined at four primary sites along the Chetco River:

Fitzhugh Bar (FPkm 15.5), operated by Tidewater Contractors, Inc.
Tamba Bar (FPkm 11), operated by South Coast Lumber Company
Freeman North Fork site (FPkm 7.5), operated by Freeman Rock, Inc.
Tidewater Estuary Bar (FPkm 3), operated by Tidewater Contractors, Inc.

Information provided by the gravel operators for mined volumes between 2000 and 2008 (the period for which actual extraction volumes for all operators is available) indicate that on average, nearly 59,000 m3 of aggregate was removed annually between the three operators, with year-to-year values ranging between 32,000 m3 (2008) and 90,000 m3 (2006) depending on permit conditions and gravel replenishment at mining sites (fig. 6).

Revised July 2012

First posted May 26, 2010

For additional information contact:
Oregon Water Science Center, Director
U.S. Geological Survey
2130 SW 5th Avenue
Portland, Oregon 97201
http://or.water.usgs.gov

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