In summer months, biological conditions and forces set the predominate ecological processes in Silver Creek and its tributaries—in winter, however, physical process dominate over biological. Winter is a stressful period for stream-dwelling organisms, especially fish. Low water temperatures slow the rate of digestion and may limit the amount of energy available for metabolism and growth, even if food is available and feeding occurs. Decreased water temperatures reduce the swimming performance of trout, which impairs their ability to escape predators. Angling is closed in Silver Creek during the winter months, but catch-and-release regulations apply in other streams in the basin.
At the onset of winter, juvenile salmonids may form aggregations in open water, especially in thermal refuges like spring inflows, or may conceal themselves in woody debris, in interstices of the substrate, or under undercut banks. Silver Creek and its tributaries offer all these conditions during winter, providing trout with ample winter cover and escapement from predators.
Winter water temperatures in Silver Creek moderate a few degrees in comparison with most trout streams in the region, but are cold enough to induce behavioral changes in trout. Water temperatures are warmer in the upper reaches near spring inflows, and are considerably cooler downstream. Of particular importance in spring-driven systems like Silver Creek is the effect and influences of ice. A floating ice cover can dramatically increase turbulent shear stress on the streambed, thereby causing peak annual sediment-transport events to occur during the breakup of an ice cover or the release of a breakup ice jam. These events often have high discharges, with gouging and abrasion of the bed and banks by moving ice. Ice in a stream channel can reduce the flow areas, increasing under-ice water velocity, scouring bed sediments, and possibly shifting the path of the deepest flow (thalweg). Solid ice is not the only condition that can alter a stream’s structure—frazzle ice can impinge flow against the channel sides, thus contributing to bank erosion.
Ice effects can occur over varying scales or time and channel length. In Silver Creek, icing is more common in the lower reaches below Kilpatrick Pond than in upper reaches, because water temperatures are warmer upstream near the spring sources. At the local scale, an ice cover over a short reach may redistribute flow laterally across the reach, accentuating erosion in one place and deposition in another. Ice may dampen or amplify erosion processes locally. Dampening effects of ice include reduced water runoff from the watershed, cementing of bank materials by frozen water, and ice armoring of bars and shoreline. Amplifying effects include accelerated erosion and sediment transport, notably during the surge of water and ice consequent to the collapse of ice jams.
Surface ice occurrences on Silver Creek are seldom and occur in localized reaches. When Silver Creek experiences ice cover, the stream tends to ice from the bottom-up, by first forming anchor ice (ice sheets attached to the substrate) and then developing frazzle ice in the water column. If low temperatures persist, thin ice sheets eventually form across the channel. While surface ice generally forms for short periods, frazzle ice and anchor ice will persist much longer. Frazzle ice is like pebbles suspended in the water column, and when working against the stream bank, it acts like sandpaper and can cause significant erosion. Stream bank abrasion by icing may explain how undercut banks are formed in Silver Creek since the stream lacks the flow velocities necessary to account for undercutting.
The detrimental effects of ice formation and sediment deposition are evident in some lower reaches of Silver Creek. Sediment deposition reduces channel capacity, and icing increases the stage (height) of the water surface in deposition areas such that stream flow overtops the stream banks in local sites, as displayed in these photos downstream of the Preserve. Sediments in this area cause winter flows to overtop banks, ice then forms on top of the bank undercuts and builds-up until the overhanging bank cannot support the weight of the ice and the stream bank collapses. This results in the loss of the undercut bank and consequently, of valuable fish habitat—it also contributes new sediments to the stream and provides platforms for encroaching reed canary grass, night-shade and other invasive species.
Silver Creek habitat is adversely affected by temperature and sediments in the summer and the winter. Continued sediment inputs and winter icing conditions will have a negative effect on channel morphology, with changes in thalweg depth and location, stream bank erosion and loss of undercut banks. Consequently, although winter is the most favorable period for stream restoration work, sediment disturbance and other potential negative effects must be carefully considered.
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