Study catchment

Choice of site

Selection of a specific catchment for use in the simulations was governed by some general requirements. In particular, it was thought it was important that it should be located in an area which had been well studied previously, thereby aiding the selection of realistic values for process rate coefficients and thresholds. It was also important that the catchment should not have been glaciated or subject to aeolian processes, as these process classes are not

included in GOLEM (Table 2.1 q.v.). Similarly, catchments including chalk or limestone strata were avoided, to reduce the influence of solution weathering. Taking these

requirements into account, and out of a number of possibilities, a catchment in the Oregon Coast Range (‘OCR’) was selected, around the headwaters of the Smith River (Figure 3.5). The OCR has been the focus of much research over the last twenty or so years, as it was believed to be an example of a region which is broadly in equilibrium with its climatic and uplift conditions (e.g. Reneau and Dietrich, 1991; Kobor and Roering, 2004). Recent research has suggested that the situation is much more complicated, with strong variations in erosion rates and uplift history across the region (e.g. Gendaszek et al., 2005; Van Laningham et al., 2006). These points were not thought to affect the usefulness of the Smith catchment and any related data sources, and all the simulations herein were therefore conducted using the Smith catchment.

The author visited the field site in 2003, during a research study visit to the U.S.A., during which various details concerning the climate, ecology and current land management of the site and its region were confirmed. The primary source data on the catchment were

obtained from the ‘CLAMS’ website7 (‘http://www.fsl.orst.edu/clams/project.htm’), and the author was granted permission by the CLAMS group administrator to download a DEM of the Smith catchment for use in this research. The DEM of the catchment provided by CLAMS was of 30 m resolution, and smoothed, so as remove pits and spikes, with the elevations being rounded to the nearest metre. Additional information on ecology and climate were provided by Professor Richard Waring, of the Department of Forestry, Oregon State University, Corvallis, who also accompanied the author on the site visit. Figure 3.5 shows both a map of central and southern Oregon and an inset, with topographic and other details, of the Smith River study catchment.

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‘CLAMS’ - Coastal Landscape Analysis and Modeling Study, a partnership supported by the U.S. Forest Service (Pacific Northwest Research Station), Oregon State University (College of Forestry), and Oregon Department of Forestry. CLAMS co-ordinates research projects and hosts databases focused on understanding

124º W 123º W

Catchment details: Elevation relative to outlet, m

Length: Width: Area: No. of cells: Elev. (outlet): Elev. (max): Relief: Mean slope (gradient): Drainge densy: Stream order: Precipitation: Est. runoff: 9.78 km 7.05 km 38.7 km2 c. 43 x 103 237 m a.s.l. 497 m a.s.l. 260 m 10º (0.17) 2.5 km-1 (est.) 4 (est.) c. 2.2 myr-1 c. 1 myr-1

Figure 3.5: Map of part of Oregon, showing location of the Smith River headwaters catchment used in this research; also plot of the DEM of the study site, with contours at 32 m

intervals, and showing additional basic details relating to the catchment.

Site description and related details

The Smith River flows into the Umpqua River near Reedsport (43º 42' N 124º 07' W), about 12 km from the Pacific Ocean. The lower reaches of the Smith are therefore tidal, and the whole river, along with the other rivers and coastal waters of this region, is

important economically for both its freshwater and anadromous fish. There has been great concern over the last fifty years to maintain fish stocks and the health of both the coastal and inland fisheries, and to protect species which have been over-fished8. Land

management practices, and particularly forestry, road building and the control of wild fires, are all factors influencing the supply of sediment to the rivers, and these topics continue to be the focus of much research (e.g. CLAMS; Lancaster et al., 2003).

The headwaters region of interest herein is located some 55 km inland at approximately 43º 45' N 123º 20' W. The climate influencing the Smith changes mainly with distance from the ocean and with altitude, but there is also a north-south trend, the northerly locations being much wetter (CLAMS). In the study site area, the climate is of a Mediterranean type, with hot, dry summers, and cooler, wet winters. Most of the rainfall (2.2 m/yr, CLAMS, 2006) falls between November and April, when stream flows are markedly higher (Waring, 2003, personal communication). Extreme summer temperatures are also possible, however, often reaching 35-40º C or more9, so there is a strong summer moisture deficit. USGS streamflow data10 were used to estimate an annual runoff for the study area catchment of 1 metre per year, which was used in all of the simulations herein. Details of the streamflow sources and runoff calculations are given in Appendix B. The dominance of the winter floods also suggested that the dominant discharge type of climate simulation used by GOLEM (subsections 2.2.3 and 3.4.2) would be appropriate for the catchment, at least as a first approximation.

The principle land use in the study catchment is forestry, and evidence from the site visit indicated a logging return period of 50 or so years (Waring, 2003, personal communication). However, some dense and tall stands suggested that parts of the catchment had probably never been clear felled. Regarding the recent logging, aerial photographs showed a number of clear cuts spread about the catchment, amounting to 10% or so of the total study area

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For example, although there is a rich Salmon fishery on the Smith, the Coho salmon has had to be protected. Similar restrictions apply to coastal and sea fish to prevent catastrophic collapse, such as happened to the pilchard stocks after the Second World War.

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(Harvey Greenberg, University of Washington, 2003, personal communication). The main tree species are Douglas Fir (Pseudotsuga menziesii) and Western Red Cedar (Thuja

plicata), with numerous examples of these trees well over 40 m tall and reaching nearly 2 m

across, measured at breast height. Other common species include Western Hemlock (Tsuga

heterophylla) and Bigleaf Maple (Acer macrophyllum), and also some Alder (Alnus), Yew

(Taxus) and Ash (Fraxinus) species. The presence of older Western Hemlock is a good

indicator that the catchment has not been subject to a major fire for some decades (Waring, 2003, personal communication). There were also many fallen trunks, in various stages of decay, lying across the forest floor, with some in or across the main river channel. The logging activity only occupies a small part of the catchment, however, and was not considered an important factor for the purposes of this research.

In the understorey, the chief shrubs are Oregon grape (Berberis aquifolium), Sword Fern

(Polystichum munitum), and Himalayan Blackberry (Rubus armeniacus), the last named

being a highly invasive, non-native species. Apart from the clear cuts, and areas in the central floodplain kept open for domestic animals, the tree cover and understorey are generally very dense and difficult to penetrate. The thickness of the tree cover and the density of the understorey together suggest that wash processes should be minimal in the catchment, and that operating GOLEM with a distinct fluvial channel function (subsection 3.4.2) would be appropriate.

The geology of the area comprises beds of the Tyee formation, a series of medium to fine grained, thickly bedded sandstones and micaceous siltstones, possibly of marine origin, with minor interbeds of tuff (Walker and Macherd, USGS, 1991). Observations of roadside cuts indicate that the bedrock is deeply weathered, to over 2 m in places, similar to the

observations made by Reneau and Dietrich (1991). These depths, together with Reneau and Dietrich’s (1991) data, were taken as an indication of the regolith thicknesses GOLEM should simulate over time in hillslope hollows and on lower slopes. Also, the absence of any limestone strata was welcome, for the reasons previously stated.

The flood plain deposits, as seen from exposed river bank sections, are two or more metres deep, and generally composed of a very fine, soft sand, lying above a coarse gravel layer of at least 30 cm thickness. However, it was not possible to explore the floodplain widely, and no cores were taken, so other deposits could well be much deeper than the ones observed. Near the location of the study catchment’s outlet, the main channel is 2½ to 3 metres wide, and it retains this width until near the source, where it declines to about 1 to 1½ m wide. In the central reaches, the river banks are generally 2 to 3 m above the river bed, and well

vegetated with trees of both deciduous and coniferous species. The banks in this zone are 6 to 7 m apart, mostly grassed, and steep or near vertical in places. Flood stage indicators (e.g. weed trapped in overhanging branches, still showing the flow direction) suggest that the previous winter’s floods had reached at least 2 m above the river bed. By contrast, on the day of the visit, the flow in the main channel was estimated to be about a 1 m3 s-1or less. These observations further emphasised the likely dominance of winter flooding.

The river bed itself comprised mostly long sections (10-40 m) of bedrock, with patches of gravel and sand, interspersed with much shorter mixed gravel and sand sections. The gravel was mostly sub-angular or rounded, the larger clasts being typically about 8 cm (A-axis) by about 3 cm (B-axis), but quite flat (< 2 cm C-axis). The smaller gravel particles were more rounded, with a distinctly rough feel. There were also a number of gravel and cobble bars, some of them embanked at one side by fallen tree trunks. In addition, in some bedrock sections, thin slabs of the bedrock had been broken clear of the river bed, and these were associated with angular cobbles and gravel downstream. These observations indicated that the GOLEM simulations should include bedrock erosion, and that the initial simulations should be able to replicate to some extent the mix of bedrock and gravel channel sections. Although the hillslopes, particularly near the source, appeared to be quite steep (c. 30º ), no evidence was found of recent landslides near the main course of the river or in the slopes bounding the floodplain. This applied also to a cleared area of similar gradients, where the trees appeared to have been felled the year before. However, there were a number of

tributaries which had steeper channel beds than the main channel, and which appeared to cut through steeper terrain. If landsliding does occur, it is more likely to do so in these

locations. Also, landsliding is widely reported throughout the OCR (e.g. Reneau and Dietrich, 1991; Kobor and Roering, 2004). Accordingly, it was decided to include shallow landsliding in the simulations with GOLEM, the steeper observed slopes (somewhat

variable, but estimated by clinometer to be about 30º) giving a good indication of the likely threshold angle for sliding. Similarly, in the clear cut areas, the hilltops were quite rounded and convex, so slow, diffusive processes were also included.

Finally, although the denudation rate near the Smith River latitudes has been estimated to be

c. 0.065 mmyr-1 (Gendaszek et al., 2005), this figure includes the much steeper and wetter areas of the North Fork Smith River, where erosion rates are much higher and landsliding frequent. The rate for the study catchment is taken to be less than this.