Stream Crossing and Time of Day

Big Hatchet Peak

The flow in streams responds to a variety of factors, especially season and recent precipitation. As one nears the head of a stream the flow rate tends to be more variable in time. This is because large streams and rivers represent the averaged discharge from a variety of small streams.
In mountainous regions a primary factor in stream flow is snow melt. A couple of graphs of flow from the North Shoshone River in Wyoming illustrate. Snow melt is most rapid when the temperatures are warm, the wind is blowing, the sun is shining, and a lot of snow remains. The only one of the factors with some subtlety is wind. High winds increase the transfer of heat from the air to the snow, thereby promoting melting. The peak flows in the stream occur near Day 5 ( about July 1) as the weather warms. After this date the amount of snow remaining declines causing the stream flow to also decline. The peak season of stream flow changes with elevation and, in the same stream system, will be generally later in higher elevation, north facing, drainages.

Figure 1. Stream flows in the Shoshone River, Wyoming. Peak runoff is around July 1 but time of day also makes a very large difference.

The diurnal (daily) pattern of runoff is related to daily sunlight, wind, and temperature patterns. On a typical day temperature increases during the day in peaks about mid afternoon. Winds follow the same pattern as temperature. Overall sunlight peaks at solar noon, but the peak sunlight on a snowfield may be at a different time of day. In general, this causes snow melt to peak in late afternoon. After melting, the resulting water must travel to the stream and down to the point of interest in the stream. This causes a time delay or lag. The length of the time lag depends upon many factors but especially the distance downstream. Near a snowfield or glacier, streamflows typically peak in late afternoon or early evening. The Shoshone River is measured at lower elevations leading to peak flows just after midnight and minimum flows in early afternoon. This is about a 6-8 hour time lag. The diurnal change in flow will also tend to increase as one approaches the melting snow. The flow in the Shoshone River in the graphs increased by about 30 to 50% during the peak time of the day. Flow near the snow is generally much more variable.

The practical implication is that during high runoff periods one may want to consider timing stream crossings to match minimum flows. The general rule is to cross in the morning; however one should observe the flow patterns in the stream and consider how far you are from the source snow melt. Early morning is not always the best time to cross as is the case with the Shoshone River at the gaging station.

Figure 2. Flows in the Shoshone River, Wyoming with expanded scale to show diurnal variations. Peak flows occur just after midnight and minimum flows are in the afternoon.