The formation of ice in rivers is more complex than in lakes, largely because of the effects of water velocity and turbulence. As in lakes, the surface temperature drops in response to cooling by the air above. Unlike lakes, however, the turbulent mixing in rivers causes the entire water depth to cool uniformly even after its temperature has fallen below the temperature of maximum density (4°C, or 39°F). The general pattern is one in which the water temperature fairly closely follows the average daily air temperature but with diurnal variations smaller than the daily excursions of air temperature. Once the water temperature drops to the freezing point and further cooling occurs, the water temperature will actually fall below freezing—a phenomenon known as supercooling. Typically the maximum supercooling that is observed is only a few hundredths of a degree Celsius. At this point the introduction of ice particles from the air causes further nucleation of ice in the flow. This freezing action releases the latent heat of fusion, so that the temperature of the water returns toward the freezing point. Ice production is then in balance with the rate ofcooling occurring at the surface.
The particles of ice in the flow are termed frazil ice. Frazil is almost always the first ice formation in rivers. The particles are typically about one mm (0.04 inch) or smaller in size and usually in the shape of thin disks. Frazil appears in several types of initial ice formation: thin, sheetlike formations (at very low current velocities); particles that appear to flocculate into larger masses and exhibit a slushlike appearance on the water surface; irregularly shaped "pans" of frazil masses that, while appearing to be shallow, are actually of some depth; and (at high current velocities) a dispersed mixture or slurry of ice particles in the flow.
The supercooling of river water, while amounting to only a few hundredths of a degree Celsius or even less, provides the context for the particles to stick to one another, since under such conditions ice particles are inherently unstable and actively grow into the supercooled water. When they touch one another or some other surface that is cooled below the freezing point, they adhere by freezing. This behaviour causes serious problems at water intakes, where ice particles may adhere and then build up large accumulations that act to block the intake. In rivers and streams, frazil particles also may adhere to the bottom and successively build up a loose, porous layer known as anchor ice. Conversely, if the water temperature then rises above the freezing point, the particles will become neutral and will not stick to one another, so that the flow will be merely one of solid particles in the flowing water. The slightly above-freezing water may also release the bond between anchor ice and the bottom. It is not unusual for anchor ice to form on the bottom of shallow streams at night, when the cooling is great, only to be released the following day under the warming influence of air temperature and solar radiation.
As stated above, frazil forms into pans on the surface of rivers. Eventually these pans may enlarge and freeze together to form larger floes, or they may gather at the leading edge of an ice cover and form a layer of accumulating ice that progresses upstream. The thickness at which such an accumulation collects and progresses upstream depends on the velocity of the flow (V) and is given implicitly in the formula:in which g is acceleration of gravity, p and p. are the densities of water and ice, respectively, h is the thickness of the accumulating ice, and H is the depth of flow just upstream of the ice cover. As a practical matter, floes arriving at the upstream edge will submerge and pass on downstream if the mean velocity exceeds about 60 cm (24 inches) per second. At certain thicknesses the ice accumulation may not be able to resist the forces exerted by the water flow and by its own weight acting in the downstream direction, and it will thicken by a shoving process until it attains a thickness sufficient to withstand these forces. During very cold periods, freezing of the top layer will provide additional strength by distributing the forces to the shorelines, so that thinner ice covers actually may be better able to withstand the forces acting on them.
As the ice cover accumulates and progresses upstream, it both adds resistance to the flow and displaces a certain volume of water. These two effects cause the depth of the river to be greater upstream, thus reducing the velocity and enabling further upstream progression to occur where previously the current velocity was too high to allow ice cover formation. This phenomenon is termed staging, by reference to its effect of increasing the water level, or "stage." In the process there is a storage of water in the
increased depth of the flow upstream, and this somewhat reduces the delivery ofwater downstream. The breakup of ice in the spring has the opposite effect—that is, the stored water is released and may contribute to a surge of water downstream.
Once the first ice cover has formed and stabilized, further growth is the same as with lake ice: typically columnar crystals grow into the water below, forming a bottom surface that is very smooth. This thickening may be predicted using equation (1), presented earlier for calculating the thickness of lake ice. An exception to this pattern arises when slightly above-freezing water flows beneath the ice cover. When this occurs, the action of the moving water either causes the undersurface to melt or retards the thickening. Since the rate at which melting occurs is proportional to the velocity times the water temperature, the ice cover over areas of higher velocity may be much thinner than in areas of lower velocity. Unfortunately, areas of thinner ice are often not apparent from above and may be dangerous to those traversing it.
In some rivers the initial formation of fixed ice takes place along the shorelines, with the central regions open to the air. The shore ice then gradually widens from the shoreline, and either the central region forms as described above by accumulation of frazil or the two sides of shore ice join.
In larger, deeper rivers, frazil produced in upstream reaches may be carried downstream and be transported beneath the fixed ice cover, where it may deposit and form
Ice forms along the banks of Vermont's Ottauquechee River. Some rivers ice over first at the shoreline, where the water is not as deep or dense. Macduff Everton/The Image Bank/Getty Images large accumulations that are called hanging dams. Such deposits may be of great depth and may actually block large portions of the river's flow. In smaller, shallower streams, similar ice formations may be combinations of shore ice, anchor ice deposits, small hanging-dam-like accumulations, and (over slower-flowing areas) sheet ice.
Ice in smaller streams shows more variation through the winter, since most of the water comes from groundwater inflows during periods between rain. Groundwater is warm and over time may melt the ice formed during very cold periods. At other times all the water in a small stream freezes, and subsequent inflowing water then flows over the surface and also freezes, forming large buildups of ice. These are known as icings, aufeis (German), or naleds (Russian). Icings may become so thick that they completely block culverts and, in some cases, overflow onto adjacent roads.
Was this article helpful?