The hydro-cryologic continuum of a steep watershed at freezeup

Abstract

• Freezeup is a very dynamic period of the year. • Ice dams store and release substantial volumes of water in the drainage system . • Stage and discharge variations can be associated with specific weather conditions. • Ice processes in small channels significantly affect the discharge of larger channels. • Monitoring simple parameters can improve our discharge estimations at freezeup. Ice processes that occur in steep (gravel bed) channels are completely different from those occurring in mild-gradient rivers. As such, the impact of ice on water level and discharge in steep streams is different than what is usually found documented for large channels. Whereas in large mild rivers, the development of a surface ice cover generates a nominal increase in stage in the order of 30%, in steep channels, the formation of series of ice dams caused the water level to rise by up to 500%. The increase in stage caused a maximum estimated ice-induced discharge depression of 50%. The ice-dam stage may represent the highest level of the year whereas the freezeup discharge may temporarily drop to its annual minimum. Ice formations significantly alter local channel hydraulic conditions (bed elevations, cross-sectional area, riffle-pool sequences, etc.) and both stage and discharge vary drastically in a short time. ‘Dynamic’ ice processes (particularly the formation of ice dams) in steep channels depend on morphology and are driven by heat fluxes into and out of the system. Based on the authors observations of many steep channels over many winters, the inter-relationships (synchronicity) between air temperature fall (and rise), ice dam formation (and breaching), water level rise (and drop) and discharge depletion (and increase) are documented here. This paper presents air and water temperatures, water stages, discharges, and ice processes taking place on an hourly basis in four channels of increasing Strahler order. The data and hydro-crologic continuum analyses presented significantly improve our understanding and hence our ability to interpret freezeup water level signals. The paper also discusses how hydrographers may use this understanding and additional data to provide better estimates of discharge in steep channels at freezeup.