Ephemeral creeks provide a vital source of water in otherwise dry landscapes. There is limited knowledge of the response of ephemeral creeks to environmental watering because research on environmental flows has focussed on permanent river systems and wetlands.
This study examined changes in instream productivity of Thule Creek and the Wakool River in southern NSW, following the delivery of environmental water from an irrigation channel. Six sites in Thule Creek, three in Wakool River, and one in Yarraman Channel were monitored for dissolved organic carbon (DOC) and nutrients once per week over 15 weeks from October 2019 to January 2020. Environmental watering of Thule Creek created a carbon-rich pulse that had positive outcomes and no observed negative outcomes. The DOC concentration of the water released from Yarraman Channel (4-5 mg/L) increased to 10-12 mg/L after travelling downstream in Thule Creek, and was as high as 80 mg/L in standing water behind a bank before it was breached to enable water to flow downstream. Dissolved oxygen concentrations dipped below 4 mg/L for a short time where DOC was greater than 20 mg/L. The flow in Thule Creek connected with the Wakool River for one week resulting in a DOC pulse in the Wakool River (DOC 10.1 mg/L) compared to the Wakool River upstream of Thule Creek (6.6 mg/L). The environmental flow was also observed to improve the condition of riparian vegetation, stimulate emergence and growth of aquatic plants, increase aquatic invertebrate activity, and resulted in over 50 colonial waterbird nesting in Thule Creek.
Our findings demonstrate that delivery of environmental water to ephemeral creeks can provide positive outcomes and stimulate productivity. Careful planning of the timing, magnitude and duration of environmental water delivery to ephemeral creeks is critical to avoid risks of poor water quality and hypoxia, particularly in hot weather.