White, P.A.; Kovacova, E.; Jebbour, N.; Tschritter, C. 2011 Waimakariri River bed and groundwater-surface water interaction. Lower Hutt, N.Z.: GNS Science. GNS Science report 2009/41 71 p.
Abstract: The hydrological characteristics of the Waimakariri River and hydrogeological characteristics of the river bed on the Canterbury Plains are crucial to the management of groundwater in Christchurch City because most groundwater in Christchurch City aquifers originates from the Waimakariri River. The Waimakariri River bed between Waimakariri Gorge and Old Highway Bridge defined in this report is approximately 91.7 km2 in area. Six reaches of the river bed are defined in this report between Waimakariri Gorge and Old Highway Bridge with boundaries of the reaches chosen considering the location of Environment Canterbury (ECan) gauging sites, river entrainment works and estimated groundwater catchment areas of Christchurch spring-fed streams south of the Waimakariri River. Groundwater recharge in each Waimakariri River reach is estimated with a simple steady-state water budget of surface water inflows and surface water outflows. For example groundwater recharge through the bed of the river is an estimated 11.7 m3/s in the four reaches between Waimakariri Gorge and Crossbank and groundwater discharge to the river is an estimated 1.5 m3/s in the two reaches between Crossbank and Old Highway Bridge. The net loss of Waimakariri River flow between Waimakariri Gorge Old Highway Bridge is estimated as 10.7 m<sup.3/s with ECan Waimakariri River gaugings measured between 1953 and 2009. Surface takes for irrigation and stock water are estimated to total 4.0 m3/s between Waimakariri Gorge and Crossbank and stream inflow to the Waimakariri River is estimated as 3.5 m3/s between Crossbank and Old Highway Bridge. Waimakariri River bed channel area in low flow conditions is estimated as approximately 10.9 km2 in the six reaches between Waimakariri Gorge and Old Highway Bridge by mapping of river channels based on ECan river cross sections. Water seepage rates from the Waimakariri River to groundwater are estimated in losing reaches at low flow in the range 38 mm/day to 230 mm/day with an average of 109 mm/day. Groundwater recharge from the Waimakariri River flows to Holocene Springston Formation sediments below the Waimakariri River bed. The direction of groundwater flow below the river bed is similar to the direction of river flow. Average groundwater flow in Springston Formation sediments under the river bed in the six reaches is estimated in the range 0.3 m3/s to 4.5 m3/s. (auth)