Coquille Subbasin Working Atlas - p11

Road Density and Road and Stream Intersections

Page 11: Road Density and Road and Stream Intersections

The Road Density and Road/Stream Crossing Density displays show the results of an analysis of large scale data focused in the East Fork of the Coquille subdivision. The road and stream data used in these analyses were photogrammetrically compiled at a scale of 1:4,800 by the BLM. The road and stream data are current as of 1988. The road coverage includes all BLM roads and most private and other public roads within the East Fork subdivision. Data are missing for a minor portion of the East Fork area. The 1:4,800 scale data shows considerably more detail than 1:100,000 scale data. In the East Fork subdivision, 1:100,000 scale data indicates a total of 262.6 miles of road, while 1:4,800 scale BLM data indicates a total of 726.4 miles of road. Likewise the 1:100,000 scale data shows 659.4 miles of streams, while the 1:4,800 scale BLM data indicates a total of 1,062.4 miles of stream. The road line data used in this analysis were derived through interpretation of aerial photography. Spatial precision of line data developed through photo interpretation can vary considerably depending on the level of control employed, and classification accuracy is affected by the skill and experience of the interpreter. Digital global positioning systems (GPS) provide new opportunities to develop accurate road line data from the ground with the aid of reference satellites in orbit around the earth. Field-based GPS data development also allows accurate classification of roads by surface type and condition and addition of other features such as culverts and bridges.

Road density

The Road Density map shows the density of roads in the North Fork of the Coquille, a subdivision of the Coquille subbasin. Road density was calculated as the total length of all known roads in a watershed unit divided by the total land area in the unit. The analysis indicates that total road density across the 153 square mile North Fork subdivision is 4.7 miles of road per square mile, varying from a low of 3.3 to a high of 6.2 by individual watershed unit.

More specific road information may allow more detailed analysis methods. A more sophisticated analysis, for example, could include weighting of roads by surface type, width, and class, and might also consider other factors such as steepness of slope and underlying geology and soils type. It might also be useful to consider analysis of exposed skid trails in addition to drivable truck roads.

[click on map for larger version]

Road and stream crossing

Table 15: Road Density and Road/Stream Crossings by Watershed in the North Subdivision
Drainage	Area sq. mi.	Road miles	Road density	Crossings Total	Crossing density
Little North Fork Coquille	3.6	22.3	6.2	43	12
Moon Creek	6.2	22.5	3.6	62	10
Alder Creek	3.8	14.3	3.8	42	11
Hudson Creek	5.8	23.1	4.0	47	8
Upper Middle Creek	6.2	25.8	4.2	50	8
Middle Lost	6.5	21.4	3.3	32	5
Vaughns Creek	2.6	10.8	4.2	5	2
Steele Creek	4.1	22.9	5.6	49	12
North Fork Coquille Creek	3.8	19.0	5.0	11	3
Whitley Reach	10.8	50.0	4.6	97	9
Upper North Coquille	13.0	60.5	4.6	91	7
Woodward Creek	6.5	28.4	4.4	72	11
Steinnon Creek	3.5	19.2	5.5	18	5
Park Creek	4.2	14.2	3.4	17	4
Lower Middle Creek	14.3	66.1	4.6	143	10
Cherry Creek	13.0	59.4	4.6	117	9
Johns Creek	2.8	16.8	6.1	61	22
Fairview Reach	15.8	94.8	6.0	174	11
Echo Valley	21.8	127.0	5.8	348	16
Echo Valley (no data)	2.2	0.0	0.0	0	0
Llewellyn Creek	2.4	7.8	3.3	10	4
Llewellyn Creek (no data)	0.2	0.0	0.0	0	0
Totals	153.1	726.4	4.7	1,488	10
Source: 1988 BLM 1:4,800 scale road and stream data.

The Road/Stream Crossing map demonstrates an analysis of the density of road and stream crossings in the North Fork subdivision. Crossing density was calculated by summarizing all points of intersection between road and stream lines within each watershed unit and dividing by the number of square miles in each unit. The primary assumptions of this approach are that (1) every intersection of road line data and stream line data represents an actual crossing on the ground, and (2) every actual crossing is represented by intersections in the line data. These assumptions, however, may not always hold true. Minor deviations in the spatial precision of the source data can result in data lines intersecting when they should not or not intersecting when they should. Given the large scale of this data, however, this method provides a reasonable estimate of the pattern of road & stream crossing density across the watershed. More accurate estimates of crossings can only be developed through intensive field surveys.

Crossing density analysis has a number of potential applications and can be used to classify watersheds by the potential for sediment introduction into streams, to schedule road survey and maintenance projects, or to prioritize restoration activities. Any interpretation of analysis results, however, must consider the limitations posed by the source data, the effects of different analysis units, and the methods used. In a more refined analysis approach, crossings might be weighted by type (bridge, culvert, ford, etc.), stream class, road surface, or steepness of slope. The ability to fashion the analysis in this manner, however, is constrained by the detail of information available in the source data.

[click on map for larger version]