Rapid accumulation of fluvially derived sands and gravels in a tropical macrotidal estuary: the Pioneer River at Mackay, North Queensland, Australia

Abstract

Sediments are accumulating rapidly in the Pioneer Estuary and adjoining foreshore. It has been estimated that approximately 4.0 × 10 6 m 3 of sediments have accumulated within the 92-year period between 1887 and 1979. Furthermore, the bar of the river, as depicted by the −3-m contour which is approximately Low Water Spring Tide level, has prograded by 600 m in the same 92 years. The Pioneer River has a catchment of only 1375 km 2 yet has recorded flood flows of up to 9800 m 3 s −1. It is a short flashy coastal stream which rises in largely granitic coastal highlands. Owing to the topography and location of the region exceptionally heavy rainfalls may occur in the catchment, the heaviest recorded being 530 mm in five hours. The flood flows produced during these heavy rainfall events supply large quantities of sand, gravel, and cobbles up to 250 mm long, to the present estuary and the bar. Extensive sand dunes have built up from these deposits on the northern side of the estuary causing restriction at the mouth. During historical time, an island composed of sands and gravels has developed within the estuary. Its growth was associated with flood events in the river. The maximum tidal range along this part of the coast is approximately 6.5 m which generates tidal flows in the estuary greater than 1000 m 3 s −1. The tidal processes have an over-riding influence on sediment distribution even in the very dynamic fluvial conditions found in the Pioneer Estuary. The substantial quantity of gravel and cobbles supplied to and present in the estuary has led to the development of gravel banks and bed armouring. The tidal currents are not competent to move these deposits but they may be moved by subsequent river floods. The deposition of flood-borne material in the estuary is affected by the state of the tide at the time of flood. Deposition occurs higher up the estuary when the tide is high; conversely coarse material can be carried right through the estuary and deposited on the bar if the tide is ebbing. The main channels in the estuary are now restrained by rock training walls. Prior to their construction it is probable that there was substantial channel migration, particularly at times of river flood. The Pioneer Estuary is an atypical macrotidal estuary containing immature fluvial sediments together with tidal depositional features. It exhibits characteristics of a wave-dominated shore line and it is considered that this is a result of the high rate of coarse sediment input. Substantial quantities of sediments of fluvial aspect have been deposited in the estuarine and near-shore environments and these sediments exhibit tidal distribution patterns. These observations are relevant to the interpretation of environmental conditions associated with the deposition of ancient clastic sediments.