Reference

Seasonal Condition Tables

Consolidated seasonal data for all documented routes. Ford depth variation, surface condition changes, obstacle frequency, and precipitation patterns by route and season. All values are derived from direct observation across multiple traversals. Route specifications list median-season values; this document provides the range.

Ford depth variation by route and season

Ford depths reported in route specifications represent the seasonal median. Actual depth varies with season, recent precipitation, and upstream conditions. The following table reports the maximum ford depth on each route by season, at the deepest crossing point.

Maximum ford depth (mm) by route and season
Route Winter Spring Summer Autumn
Western Coast 680 680 520 600
Basin and Range 300 650 350 400
Divide Transit 200* 750 400 350
Great Plain 400 850 450 500
Eastern Seaboard 600 950 700 650
Gulf Lowland 500 1,300 900 700
* Divide Transit winter fords may be frozen or covered by snow. Depth measurement reflects ice-free conditions. Fords above 2,800 m may be frozen solid from late autumn through mid-spring.
Surface condition by route and season

Surface conditions on soil, grass, and clay substrates change significantly with season. Asphalt and rock surfaces are minimally affected. The following table indicates the primary surface concern by route and season.

Primary surface condition factor
Route Winter Spring Summer Autumn
Western Coast Rain softening (S third) Erosion gully deepening Firm throughout Fog (headlands)
Basin and Range Alkaline flat wet risk Mud in eastern segment Heat / tire pressure Stable
Divide Transit Snow / ice above 2,800 m Scree instability, thaw Stable (thunderstorm) Early snow at passes
Great Plain Frost heave (W half) Saturated clay / riverbed Tall grass / visibility Stable
Eastern Seaboard Ice (N third) Soft soil / flooding Vegetation density Storm surge (S third)
Gulf Lowland Stable (driest) Flood / soft substrate Storm surge risk Storm surge (early)
Precipitation by region and season

Average monthly precipitation affects ford depth, surface bearing capacity, and visibility. Values are estimated from observed conditions and may not reflect historical averages. Precipitation patterns may have changed.

Estimated average monthly precipitation (mm)
Region Winter Spring Summer Autumn
Western coast (N) 180–250 80–120 20–40 100–160
Western coast (S) 60–100 30–50 5–15 30–60
Intermountain / basin 15–30 15–30 10–25 15–30
Continental divide 40–80* 50–90 60–100 40–70
Interior lowland (W) 15–25 40–80 50–90 30–50
Interior lowland (E) 40–70 80–120 80–110 60–90
Eastern seaboard (N) 60–100 80–120 90–130 80–110
Eastern seaboard (S) 60–80 80–110 150–200 80–120
Gulf lowland 60–100 100–150 120–180 80–120
* Continental divide precipitation above 2,800 m falls predominantly as snow from late autumn through mid-spring. Snow water equivalent is approximately 100–200 mm per metre of snowpack depth.
Freeze-thaw and snowmelt timing

Freeze-thaw cycles affect road surfaces (frost heave), ford conditions (ice), and slope stability (permafrost thaw, scree loosening). Timing varies by elevation and latitude.

Approximate freeze-thaw timing by elevation
Elevation First freeze Last freeze Snowmelt peak
<500 m (N regions) Late autumn Mid-spring Early–mid spring
<500 m (S regions) Rare / none n/a n/a
500–1,500 m Mid-autumn Late spring Mid-spring
1,500–2,500 m Early autumn Late spring Late spring–early summer
2,500–3,000 m Early–mid autumn Early summer Early–mid summer
>3,000 m Early autumn Mid-summer Mid-summer
Snowmelt peak is the period when ford depths increase most rapidly at the corresponding elevation. The snowmelt peak at 3,000 m produces maximum ford depths at all lower elevations with approximately one week of delay per 500 m of elevation loss, as meltwater travels downstream.
Optimal traverse windows

Based on the combined seasonal factors, the following windows present the most favorable conditions for each route. These are not requirements; all routes are traversable in all seasons with appropriate vehicle specification. The windows indicate when the documented specifications are most likely to match observed conditions.

Recommended traverse windows
Route Optimal window Most demanding season
Western Coast Late spring – early autumn Winter (rain, soft soil)
Basin and Range Autumn Spring (snowmelt, mud)
Divide Transit Mid-summer – early autumn Spring (snowmelt, rockfall)
Great Plain Autumn Spring (saturated riverbed)
Eastern Seaboard Late spring – early summer Autumn (storm surge, S third)
Gulf Lowland Winter Spring (flood, soft substrate)
Revision

All data derived from direct observation on documented routes. Ford depths measured at the documented crossing points. Precipitation estimates based on observed conditions during traversals and are subject to annual variation. Freeze-thaw timing approximate; actual dates vary by year. Optimal windows represent the consensus of all available traverse data.