Coquille Subbasin Working Atlas - p3
Terrain
Page 3: Terrain
Page 6: Vegetation and Land Cover
Page 8: Water Use and Availability
Page 9: Fish Distribution & Habitat
The elevation of the Coquille subbasin ranges from near sea level feet at the mouth of the Coquille River where it meets the Pacific Ocean, to 4,075 feet at Ophir Mountain in the southern end of the subbasin at the headwaters of Rock Creek.
The three-dimensional map of shaded relief shown on the atlas cover was developed from 73 meter digital elevation model (DEM) data. This data consists of a grid of cells, each representing an area of 5,329 square meters (about 1.3 acres) on the ground. Each cell is assigned an elevation value which represents an approximation of the average elevation for that area. Shades of gray have been assigned to ranges of elevation values in this terrain model to illustrate the variation in terrain. Dark shades depict the lower elevations. Mid to high elevations are represented in increasingly darker shades.
The accuracy of any elevation model is limited by the resolution of the source data which restricts the scale at which it can be accurately displayed and analyzed. The 73 meter data used in this model is fairly coarse, and was developed from a combination of three sources of data with resolutions of 20, 30, and 70 meters. No single source of data covers the entire Coquille subbasin.
Higher resolution data is more accurate and can be used at larger scales of analysis. Twenty meter data has over 12 times the resolution of 70 meter data, with each cell representing about one tenth of an acre versus approximately one and one fifth acres. Elevation models can be manipulated to develop slope, aspect and other terrain related data.
Stream gradients
The 73 meter elevation data was used with the 100,000 scale stream data to develop estimates of stream gradient profiles for the major streams in the Coquille basin. This was accomplished by combining the two data and deriving an average elevation and horizontal distance for individual stream segments. Estimated profiles are show for the North, East, Middle, and South Forks of the Coquille River in Figures 3–6. As can be seen in these four charts, many downstream segments appear to have higher elevations than their upstream counterparts. This anomaly is particularly apparent in the lower reaches of the North Fork in Figure 6. This is due to the coarseness of the elevation data, the small scale of the stream data, and the high variability in terrain common to river systems. Despite the error, these profiles provide a reasonable view of the variation in gradients between these four streams.
Figure 4: Gradient of East Fork of the Coquille River 
Figure 5: Gradient of South Fork of the Coquille River 
Figure 6: Gradient of North Fork of the Coquille River 
Figure 7: Gradient of Middle Fork of the Coquille River 
