Hope for the best, plan for the worst: Managing sediment input in the upper catchment whilst preparing for avulsion at the mouth. (#82)
The George River in north east Tasmania runs from west of Pyengana to St Helens, where it flows out onto a large delta within Georges Bay. Previous research shows the delta, on which the township of St Helens it built, is Holocene in age. The George River and the Golden Fleece Rivulet to the south have both contributed to delta progradation during the Holocene and the George River has occupied several courses on the delta over that time. Currently, the George River occupies a course at the northern boundary of the delta; however, increased sediment accumulation in this section of the channel is causing rapid infill and avulsion to a new course on the floodplain is becoming imminent.
Sitting in a granitic catchment the George River is naturally subject to high sediment loads. However, historic hydraulic mining and land clearing have increased sediment loads substantially. The George River consists of alternating floodplain and gorge sections. As such, eroded sediment is forced rapidly through gorge sections and stored temporarily in the valley expansions, either as splays on the floodplain or sand slugs in the channel. Increased bank erosion due to a lack of vegetation or willow outflanking is leading to a substantially increased rate of sediment moving to the mouth, instead of being stored in these sections. Combined with substantial willow infestations in the lower reaches the river mouth is soon set to fill completely.
Management options to reduce sediment load include weed management, riparian revegetation and stock exclusion. However, the channel is very close to the threshold required for avulsion and the scale of bank stabilisation and revegetation required is vast. Whereas, managing sediment in the upper catchment is strongly recommended, preparing for avulsion at the mouth is essential to ensure a stable new course for the George River.