BAR MIGRATION IN DRYLAND RIVER WITH HIGH-DISCHARGE VARIABILITY, THE DEGREY RIVER, NORTHWESTERN AUSTRALIA

Research covering dryland subtropical river systems with highly variable discharge rate is limited. These rivers are shaped by substantial flash flooding events driven by cyclones. These conditions apply to the DeGrey River in Northwestern Australia, and this study presents findings on the fluvial bar processes of this flash-flood-driven river. Aerial imagery from Google Earth over a 36-year study period from 1984 to 2020 was utilized to observe bar-migration trends. In addition, LiDAR captured in September of 2021 provided detailed recent topography of the study area.

Six bars were identified in the channel and mapped in time steps over the target interval. Bar types included laterally growing bars, downstream migrating bars, and mid-channel bars. The growth of these bars was quantified over the 36-year study period. Yearly bar migration rate was also compared to the max discharge of that year.

The results show that the growth and migrations of these bars is trackable, and they persist through these large annual floods. The channel fill was not evacuated by large floods, and in-channel bars persist between and through large flood events and behave more similarly to a river with more stable flow conditions. This counters to other similar and proximal rivers where fluvial channel-fill structures are removed by these seasonal cyclone-driven floods. The yearly migration rate of each bar resulted in a positive linear trend when compared to the max discharge for that year showing bar migration rate is tied to discharge magnitude. While the strength of the positive trend varies between the individual bars, the data shows a larger max discharge for a given year will produce a greater amount of downstream bar migration in this river. The collective downstream migration of these bars has averaged to 113 meters per year with this being an average of 38 meters per year for each individual structure. However, each of these channel bars experience their own unique rates of migration which even include a few sporadic years of backward, upstream migration.