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Predicted Stream Crossings as an Indicatorof Road Hazard
Name of indicatorPredicted stream crossings
Questions potentially addressedAQ (1) How and where does the road system modify the surface andsubsurface hydrology of the area? AQ (2) How and where does theroad system cause surface erosion? AQ (4) How and where do road-stream crossings influence local stream channels and water quality?AQ (9) How does the road system alter physical channel dynamics,including isolation of floodplains, constraints on channel migration,and the movement of large wood, fine organic matter, and sediment?AQ (10) How and where does the road system restrict the migrationand movement of aquatic organisms? What aquatic species areaffected and to what extent? How and where does the road systemcontribute to fishing, poaching, or direct habitat loss for at-riskaquatic species? AQ (13) How and where does the road systemfacilitate the introduction of non-native aquatic species?
Description of indicatorPredicted stream crossings are the points where roads and streamsintersect, based on GIS coverages or topographic maps.
Units of indicatorThe road-stream intersection points can be attributed with streamorder, slope position, slope class, or any available GIS polygoncoverage.
ScalesMay be useful at all scales.
Related IndicatorsPredicted road-stream crossings are primarily a function of roadlocation. Areas of high road and stream density usually have a higherdensity of predicted road-stream crossings. There may be areas whereroads on ridges are above stream channels and have few intersectionswith streams.
UtilityPredicted road-stream crossings provide an estimate of the locationand number of actual crossings. They can also be attributed withother data from the road or stream layers or from polygons.
AcquisitionRoad-stream intersections are produced by intersecting the road andstream layers and putting the intersection nodes into a point coverage.
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Data needsMust have a roads and streams coverage for the GIS process. Can bedone manually by using topographic maps with roads and streams.
Accuracy and precisionThe accuracy of the stream coverage is critical to producing a usefulpredicted stream-crossing coverage. Streams delineated ontopographic maps typically underrepresent the actual streamchannels. Crenulated contour coverages may overrepresent streamchannels, depending on the rules used and the individual crenulator.Stream channels delineated by using GIS flow-accumulation programsmay not reflect actual stream channels.
DurabilityWill change over time if roads are dropped or added to the roadscoverage.
Monitoring valueProbably not useful for monitoring, although if roads are dropped fromthe system there will be fewer predicted crossings.
LimitationsThe primary limiting factor is the accuracy of the stream-channelcoverage. If it overestimates channels, the predicted number ofcrossings will be high. If the only stream coverage is from the USGSlines on topographic maps, there will probably be significantly fewerpredicted crossings than actually exist.
Typical availabilityWherever roads and streams coverages exist.
Where applicablePredicted stream crossings can be used as an initial estimate of thenumber and location of crossings. By intersecting other coverages,such as slope position, it may be possible to make an estimate of fillvolume or other factors associated with high risk crossings.
ExamplesAn intersection of roads and blue-line streams for the Bluff Creekwatershed (figure 2-39) has 36 intersection points (predictedcrossings). The intersection of roads and stream channels from acrenulated-contour coverage (figure 2-40) has 469 predicted crossings.This is more than 13 times the number of crossings predicted by usingthe blue-line streams. The Bluff Creek watershed was subdivided intoan upper, middle, and lower areas that are about equal. The crossingdensity (crossings/mile of road) was calculated for each area (figure 2-41). The upper area has a lower crossing density than the middle andlower because most of the roads in the upper area were built onridges. Many of the crossings in the Bluff Creek watershed wereinventoried and compared to the predicted crossings (figure 2-42).
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Predicted crossings that were on roads that were not inventoried arenot shown. The remaining predicted crossings more than 25 metersfrom an inventoried crossing were assumed not to be actual crossings.A total of 134 predicted crossings were more than 25 meters from aninventoried crossing. There were 358 predicted crossings and 224inventoried crossings.
Development needsThe macro for intersecting road and stream coverages to predictstream crossings will be incorporated in a software package. It willalso allow attributes from either the road or stream coverage to beapplied to the crossing points.
Tools referencesHydrologic Condition Assessment Tools – Module of Indicators forRoads Analysis (See Appendix 3)
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Road systemBlue-line streams
xz Road blue-line stream intersections (36)Boundary
N
EW
S
1:120,000
ExampleRoad-Stream Crossing - Layer
Watershed - ScaleBluff Creek WatershedOrleans Ranger District
Six Rivers National Forest
1 0 1 2 3 4 Miles
Figure 2-39. Intersections of roads and blue-line (USGS) streams from the Bluff Creekwatershed. There are a total of 36 predicted road-stream crossings.
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xz Road crenulated-stream intersections (469)Road systemCrenulated streamsBoundary
N
EW
S
1:120,000
ExampleRoad-Stream Intersections - Layer
Watershed - ScaleBluff Creek WatershedOrleans Ranger District
Six Rivers National Forest
1 0 1 2 3 4 Miles
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Figure 2-40. Intersections of roads and crenulated-contour coverage for the BluffCreek watershed. There are 469 predicted crossings compared to the 36 predictedby using the blue-line streams.
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2.91
2.38
0.71
Predicted crossing density (crossings/square mile)Upper Bluff (0.71)Middle Bluff (2.38)Lower Bluff (2.91)
xz Road crenulated-stream intersections (469)Road systemCrenulated streams
N
EW
S
1:120,000
ExampleRoad-Stream
Crossing Density - LayerWatershed - Scale
Bluff Creek WatershedOrleans Ranger District
Six Rivers National Forest
1 0 1 2 3 4 Miles
Figure 2-41. Predicted road-stream crossing density for the upper, middle, and lowerareas of the Bluff Creek watershed. The density in the upper area of the watershedis 0.71 crossings/mile compared to 2.91 in the lower area
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ExampleRoad - Stream Crossing - Layer
Watershed - ScaleBluff Creek WatershedOrleans Ranger District
Six Rivers National Forest
1:120,000
N
EW
S
Road systemCrenulated streams
Predicted crossings on inventoried roads#S Less than 25 meters from an inventoried crossing#S More than 25 meters from an inventoried crossing
Ñ Inventoried crossingsBoundary
1 0 1 2 3 4 Miles
Figure 2-42. Comparison of predicted road-stream crossings (from intersection ofroad and crenulated-contour coverages) and inventoried crossings. There are 1.6times as many predicted crossings than inventoried.
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