Dear Doc Hydro: A local conservation district is working with riparian land owners to implement riparian and stream channel improvements. We have just experienced a significant rain on snow event. Ice damming was common and caused overbank flooding. Temperatures prior to the event were below zero for some time so formation of ice was common. Some land owners contend that the presence of riparian vegetation contributes to the build up of ice and therefore to the potential higher water when the dam breaks. From a technical viewpoint, what is the function and occurrence of ice formation on contrasting good and poor quality stream systems?

In summary, deep streams bordered by thriving healthy riparian areas (i.e., trees and shrubs) are less likely to have ice build-up problems than shallow, exposed streams.

The formation of ice in a stream involves the same basic processes of heat exchange, nucleation, and growth that lead to the formation of lake ice. The major difference is that with streams, the fluid is in motion and therefore, the evolving ice crystals must oppose the forces of the moving water if they are to grow and ultimately form a solid ice cover.

Surface flow velocity plays an important role in ice formation. Conditions most suitable for ice formation occur where flows are laminar, permitting supercooling of the surface. Flow velocities less than 0.5 feet/second are common along shores of most reaches and may also occur over the entire stream. Under these conditions, a surface layer of supercooled water can develop on the low velocity flow. Once nucleated, the ice sheet can quickly spread across the entire surface. The first ice normally forms along the margins of the stream, near the banks, although it can be initiated midstream and progress simultaneously upstream, downstream, and toward the shore. Ice growth rates under slow velocity conditions can be extremely fast and there are reports of a reach of river forming an ice cover over a 5 mile distance in less than 15 minutes.

In turbulent sections of the stream, water supercooled at the surface constantly mixes with the underlying flow and ice formation slows. The entire cross section must reach 0°C and then with a slight degree of further cooling (about -0.1°C), small ice particles, called frazil ice, begin to form. Because the supercooling that controls frazil formation is not great, slight changes in the thermal regime, such as frictional heat produced by rapids, can appreciably affect frazil ice production.

Hydrometeorological conditions most favorable for ice production include cold clear nights with large heat loss by long-wave radiation and a strong wind accompanied by cold, dry air which produces large convective and evaporative heat losses. It is fairly typical for frazil ice formation to follow a diurnal cycle characterized by rapid growth at night and cessation during the day. Given adequate temperature conditions, frazil ice continues to grow and a stationary ice sheet may be formed either by having border ice completely cover the stream or by some mechanism which produces a physical halt of the ice flow
in the stream.
When the stream is shallow and the turbulence of the water is high enough to bring supercooled water to the bottom, ice may attach to underwater objects and produce anchor ice. Given suitable hydrometeorological conditions, anchor ice can form blankets of ice on the stream bottom. Anchor ice most often forms at night. Once the stream begins to warm slightly, or when solar radiation penetrates to the bed, the bond between the bottom and the anchor ice disintegrates and the ice releases. Floating anchor ice is readily discernible because it often contains embedded gravel and other bed material.

Once a surface ice cover is established, it thickens as the freezing front migrates vertically downward into the water column. A surface snow cover tends to retard the growth of static ice by slowing heat loss to the atmosphere. In high-northern climates, ice growth and low winter flows can be severe enough to cause complete freezing of the bed of small streams.

Break-up of ice occurs as ice decays due to thermal degradation of the ice cover. Break-up at a site depends on a number of variables including cover thickness, ice strength, river geometry, flow velocity, and stage. The severity and pattern of breakup is influenced by the alignment of the stream relative to the local climate. On streams in which snowmelt, runoff, and ice break-up proceed downstream with the seasonal advance of warm weather, the ice jam and flood risk is heightened because the spring flood wave is always pushing against an intact ice sheet. On streams flowing in a direction opposite to that of regional warming, risk is reduced because thermal ablation of the ice pack greatly reduces the probability of ice jamming.

Collapse of an ice jam and the release of water in channel storage can produce a surge characterized by dramatic increases in downstream water levels and velocities. The magnitude and rate of the surge has a far more catastrophic flood potential than that possible under similar open-water flow conditions. The abrasive action of rapidly moving break-up fronts can be an important modifier of channel beds and banks, particularly in alluvial rivers.

So what does all of this have to do with good or bad stream conditions or the vegetative state of the riparian area? Specific literature linking ice to channel and riparian condition is rare, but inferences can be drawn from the physical processes described above.

The larger the surface area to volume ratio, the more quickly a stream exchanges heat with the atmosphere. Hence, given the same meteorological conditions, wide shallow streams cool most rapidly while deep rivers are usually the last to freeze. Since excessive grazing often result in a widening of the stream channel, we can infer that ice problems are exacerbated.

We also know that open water streams without forest cover freeze earlier because the missing canopy allows for outward radiation from the water surface, especially at night. The presence of a canopy reflects heat radiation back to the stream surface, thus keeping the stream warmer relative to an exposed reach. From this we can infer that well-vegetated streams bordered with riparian trees and shrubs will tend to have less severe freezing than stream reaches devoid of streamside vegetation.