Assessment of water quality risks associated with managed flooding of a large-scale floodplain-wetland complex

"July 2010".

Project Number: Chowilla floodplain: Water quality risks associated with Schedule A (2009/10 watering of the Chowilla floodplain at Coppermine floodplain and an in channel water level rise at Punkah Creek) - M/BUS/335.

MDFRC item.

This project had three primary objectives: (i) Investigate dissolved oxygen and nutrient release dynamics at a large-scale floodplainwetland complex (325 ha) where management intervention (pumping and ponding of water on the floodplain) has been undertaken. (ii) Investigate the potential for a conservatively managed return of flood water from the floodplain to the anabranch system to produce a measurable nutrient pulse in the receiving waters. (iii) Investigate the potential for water level manipulations within a shallow anabranch creek to lead to the onset of strong, persistent thermal stratification. i. The Coppermine waterhole-floodplain complex is a moderately deep (< 2 m) retaining basin and adjacent floodplain depressions that fills through an ephemeral channel leading from Monomon Creek into the eastern end of the wetland. During high flows, the wetland-floodplain complex drains into Chowilla Creek at multiple locations. The management intervention involved pumping water from an anabranch creek into the Coppermine waterhole and the adjacent lignum shrubland. The information generated on dissolved oxygen depletion rates is presented in order to provide managers with additional insight into the minimum exchange rates required to maintain water quality standards during managed floods. The results of this study support previous estimates of water exchange required to maintain dissolved oxygen above a threshold of 4 mgL-1. ii. Ponding of water on floodplains has the potential for negative water quality outcomes and excludes the potential for nutrient transfer from the floodplain to the river. This study demonstrates that a substantial load of nutrients is contained in the water ponded on the floodplain. The simple modelling approach taken here suggests that a conservatively managed return of flood water to the anabranch system would result in measurable nutrient pulse that could be detected in Chowilla Creek without contributing a problematic seed source of cyanobacteria to the river channel. It is widely assumed that transfer of nutrients stored on the floodplain to the river channel is an essential process for riverine function (Robertson et al. 1999; Francis and Sheldon 2002). Therefore, it is anticipated that ecologically relevant productivity pulses may be able to be generated via managed inundation and subsequent release of water from relatively small areas of floodplain. iii. Following observations of a positive response in River Red Gum trees lining the Punkah Creek during a brief rise in water level during an in-river flow spike, the utilisation of a temporary structure (aquadam) within Punkah Creek was trialled in an effort to induce variation in water levels within Punkah Creek independent of flows within the River Murray. Water quality data collected from logger stations within the creek demonstrate a marked increase in the strength of thermal stratification that appears to be tightly coupled to the increase in water depth. This suggests that there was drop in water velocity (and potentially downstream water exchange) during the filling phase while the weir pool created by the temporary structure filled. Maintenance of flow velocity and water exchange to maintain water quality is a critical area of concern that must be addressed during managed floods, particularly during operation of the regulator being constructed on Chowilla Creek. Failure to maintain flow and water exchange is likely to produce negative water quality outcomes that may have long-term or irreversible effects.