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Any discussion of proper boating technique needs to begin with practical hydrology, as paddling on whitewater requires working with the river current rather than fighting it. It is essential to understand the factors influencing water flow to have a successful, or perhaps equally important, to have an enjoyable run on whitewater. Factors required for the formation of rapids are also included to facilitate the detection of rapids and falls even though such hazards are usually more appropriately recognized by direct audible and visual warnings. When traveling down an unknown river, even the flatwater boater must know how to spot rapids and falls so that he can avoid them as required.

Water Cycle

The influence of the environment on stream flow is best understood by considering the movement of water from the atmosphere to the earth and back to the atmosphere again. This is referred to as the water cycle.

Water that has fallen on a watershed travels over three routes. Part runs over the surface of the land directly into rivers as surface runoff; part is retained by the soil where it is absorbed by plants or lost by evaporation; and part seeps down into the ground and moves laterally as ground water until it is discharged into springs, rivers, and swamps as groundwater runoff. In general, the greater the amount of water that follows the subsurface route as ground-water runoff, the higher the flow during dry periods and the more likely the stream will have sufficient water to permit canoeing most of the year.

Seasonal Variations

Variations in discharge (water flow rate) on undammed rivers are caused by variations in precipitation, snow melt, and evaporation. Highest discharge normally occurs during the spring snowmelt. The maximum discharge depends on the water content of the snow, amount of frost in the ground at the time of snowmelt, rate of melting, permeability of the soil, ground cover and slope of the land surface. Discharge usually declines during the late spring, reaching a minimum during the summer months when growing plants use up (transpire) large quantities of water. This is also the period when relatively high temperatures cause increased evaporation. Also, since the ground is not saturated, rain water sinks in rather than runs-off. Killing frosts in the fall reduce transpiration and usually cause a slight increase in discharge. Much of the winter precipitation is in the form of snow and does not reach the river until the following spring. Thus, discharge declines during the winter months. Unusually intense rains at any season may cause high rates of discharge.

Seasonal variations in discharge are common to most rivers, but the magnitude of the variations ranges widely. These variations generally result from differences in the relative amounts of water derived from surface runoff and ground-water discharge. The quality of the watershed (drainage area) rather than its size determines the ground-water/surface-water relationship, and this in turn determines whether a river will be runnable at most times of the year.

Aspects of the watershed such as permeability of the soil, topography of the land, and amount and type of vegetative cover determine the relationship among surface runoff, ground-water runoff, and evaporation. Air temperature directly influences the rates of evaporation and transpiration, which are highest in the summer. Higher rates of evaporation are found in areas with relatively impermeable soils such as clay or in areas where there is little soil above the bedrock. Highly packed soils hold greater amounts of moisture by capillary action than do the more porous sandy soils and consequently more water is available for evaporation and transpiration.

The nature of the glacial deposits and the type of soil in a watershed affect the rates of infiltration which in turn determine the route by which precipitation reaches a river. In areas of silt, clay, or exposed bedrock, there is little permeability and most of the water travels to the river as surface flow. On the other hand, in regions composed chiefly of sand, gravel, and other porous materials, much of the water travels as ground-water runoff.

Surface runoff reaches the river a short time after rains (or snowmelt), usually within a few hours. After reaching the peak discharge, surface runoff declines rapidly. Ground-water runoff is slower to reach the stream and slower to decline after reaching a peak. It is ground-water runoff that keeps the river flowing during rainless periods. In general, the greater the component of groundwater discharge the more uniform the flow.

Formation of Rapids

Rivers differ from man-made channels of uniform cross section. The width, depth, gradient, and composition of the riverbed varies (sometimes dramatically) at different sites along the river. Rapids, falls, and other potential hazards may result from such variations even on rivers that consist essentially of flatwater. A practical knowledge of river hydrology will enable you to anticipate hazards likely to be encountered in river running. Hopefully this information will enable you to prepare accordingly.

There are three major components of stream flow— discharge, velocity, and stage. Discharge is the volume of water moving past a given cross section of river per unit of time. The units generally used to express this measurement are cubic feet per second (cfs). Velocity (river current) is the speed of water movement (in feet per second). Stage is the height (in feet) of the water level above an arbitrary zero point. All of these components are interrelated; when water levels are up, so are the volume and speed of flow.

The most practical method used to determine discharge is to measure the stage of the river and to use a conversion table that relates stage to discharge. This relationship depends on the configuration of the stream bed and differs for each river and for each section of the same river.

The velocity or current speed at a given discharge depends on two factors—gradient and cross-sectional area of the stream bed. The profile or gradient of a river refers to its fall in feet per mile from source to mouth or within given sections. The steepness of gradient influences canoeability chiefly by modifying the velocity of stream flow. Where the gradient of a river increases, the velocity of stream flow increases.

Normally the profile of a river is relatively steep in the headwaters and relatively flat near the mouth. However, this general relation is modified within any given section by the geology of the river basin. Where topography permits, local steepening of gradient occurs where rivers cut across exposed bedrock or beds of stony outwash because these materials are more resistant to erosion than the rest of the riverbed. Beds of dense clay in outwash deposits may also resist erosion and cause a steeper gradient.

In addition to gradient, the second factor that determines current speed is the cross-sectional area of the river channel. For a given flow rate (discharge), velocity increases when the cross-sectional area of the channel decreases. This occurs either when the river becomes shallower or when the river narrows. The increased velocity that results from a shallow or narrow channel frequently results in riffles or rapids. In regions where the channel is deep, stream bed materials are of little significance to the boater. However, rapids are usually found where the river passes over shallow regions of erosion-resistant outwash or exposed bedrock. Riverbeds having large boulders with water moving at a relatively high velocity produce rapids with a high hazard rating.

Variations in current speed are likely within any given channel. Current speed is influenced by obstacles such as rocks and vegetation found along the river-bed which slow down the flow rate. Therefore the current is usually fastest in the middle and slowest along the sides and bottom. The current ranges from more than 4 feet per second in the upper center of the channel to less than 1 foot per second near the bottom and sides of the channel.

Another generalization that frequently holds is that the current is usually greatest at the outside of turns. Fast current tends to scour out the outer channel, so that's usually where the water is deepest.


River reading involves recognition of obstacles and evaluation of the water flow so that an appropriate course can be chosen. A good route through obstructions in a rapids is called a chute.

Chutes are usually characterized by smooth water in which surface riffles suggest the letter "V" with the point downstream. Conversely, surface ripples that suggest the letter "V" with the point upstream indicate a rock at the point of the "V" and must be avoided.


Rocks in the river bed, unless they protrude above the surface, are recognized by the nature of the turbulence downstream. The turbulence (hydraulics) below rocks is related to the gradient, discharge, and size of the obstruction.

During periods of low discharge, large rocks protrude well above the surface and are readily obvious from upstream. Eddies occur below both of these obstructions. An eddy is an area immediately downstream of an obstacle in a river where the current is relatively calm or may actually flow upstream. The eddy results from water filling the void behind the obstacle as the main current flows past. Eddies provide a place for rest stops in the midst of even the most difficult rapids.

Rocks submerged just below the surface of the water can be recognized by a smooth hump that is formed by the fast-moving water as it follows the contour of the rock. This smooth hump of dark water is often followed by white turbulence downstream. Be careful since the turbulence may be several feet downstream of the rock. Such rocks are hard to spot. In slow-moving water, the hump may not show. In commonly paddled areas these rocks can sometimes be identified by their shiny coat of aluminum left by previous boaters.

Souse Holes and Backrollers

The water flows over a rock it piles up on itself, creating small waves. If the water falls steeply enough, the first wave may curl back on itself creating a back wave or backroller. Water will also flow around the rock creating an eddy. The combination of eddy and backroller creates the hydraulic known as the souse hole. In a souse hole the water enters the depression from all sides— from upstream over the rock, from downstream (backroller), and from the sides (eddy action). At low , the mini-souse hole is merely a mildly turbulent eddy. Large souse holes can be hazardous. Experts and only experts should even approach let alone willingly enter one.

Standing Waves

Standing waves result from the slowing down of a fast current and are usually associated with deep water. Because of the increased depth required to produce standing waves, they provide a good rock-free ride through a rapids. Occasionally troublesome insufficiently submerged rocks may be concealed among the standing waves. Their presence can usually be identified by the irregularity of the waves.

Standing waves also form downstream of any constriction in the width of a river channel. For example, standing waves are frequently encountered at a point where the river widens below a narrow canyon. Conditions necessary to form standing waves do not require much gradient if there is adequate discharge.

River reading is an art that must be mastered slowly and sometimes with much bumping and scraping. The topography of the river bed is revealed much more clearly from the bottom of the rapids than from the top. It is often helpful to look back upstream after running a rapids and try to relate these two completely different views or to scout a rapids from below before running it.