The Mighty Ord

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When I left the Coastal Studies Institute (CSI) at Louisiana State University in 1967, I never thought I would work on another project with that group. It was a pleasant surprise to receive an invite in early 1971 to join Don Wright in a study of the Ord River estuary. This was part of research being undertaken by CSI of deltaic systems sponsored by Geography Programs of the US Office of Naval Research (ONR). It was another case study of the interaction of various process controls in delta geomorphology, but this time in an area where tides play a dominant role. I jumped at the chance to be involved; I had no experience myself in working in such an area, and it was a good opportunity to work with Don who at that stage I knew more by reputation. Moreover, it was great to get the ONR supporting field work in Australia in ways that would not otherwise have been possible.

The project had three components: one, was to obtain a general understanding of landforms, sedimentary environments and processes along the stretch of coast between Darwin and the Cambridge Gulf; two, was to look at form-process interactions responsible for the morphologic characteristics of tide-dominated channels; and three, to look at the relationship between mangrove communities and landforms in a high tidal range (c.6m), seasonally dry environment. Field work took place during the dry season of 1971 and the wet season of 1972. We were visited briefly by Jim Coleman from LSU, but otherwise left to our own devices. Subsequently we published several papers and field reports and have used the experience from this project in other studies.

Don and I drove from Darwin to Wyndham carting wave tide recorders, current meters, drogues, and a salinity meter. We also had a range of survey equipment and drilling gear and a useful set of geologic maps, bathymetric charts and air photos. The US Navy through ONR and CSI had provided the two of us with an incredible opportunity to explore and test hypotheses on a little known estuarine system, and we did not have to undertake any OH&S assessment!

The drive to Wyndham in itself was an education as neither of us had seen such landscapes. The baobab tree seemed to fascinate Don more than me for some strange reason. At the time of our visit, there were attempts to farm cotton and we observed the impacts of pesticides on fish at downstream locations. However, we were both thrilled to see the bold Precambrian rock outcrops of the east Kimberleys surrounding the turbid waters of the West Arm of the Cambridge Gulf, and the mud skippers on tidal flats at low tide. Stark outlines of salt deposits representing different inundation levels of high tides on the vast, bare tidal flats were very striking. And it was all so dry! We could drive our little truck across the flats to visit sites along the King River and the southwest bank of the Ord River. At our disposal was a 10m, flat-bottomed barge operated by a frustrated skipper who had recently been stopped from crocodile hunting. His barge was ideal for use in shallow parts of the estuary where grounding could be a twice-daily event. The skipper could not always contain himself as crocs hiding in stranded pools were fair game although none were ever hit to our knowledge; we amused ourselves on these occasions by wandering off into the mangroves or measuring bedforms (huge mega ripples) on sand flats. When I returned in the wet season, there was no barge and no truck that could cross wet mudflats. But I could still drill holes, and a helicopter was available to access remote areas that we did not get to in the dry.

An important part of the landform story of the Ord River is the relation of dry season tidal exchange and river discharge of short duration during the wet season. Many questions arose about the behaviour of tidal processes operating along the length of the lower 65km of the river. One was whether there was evidence of the river alluvium aggrading over tidal flats (Search, 1972, vol. 3, 339-341). However, our focus was on tidal processes extending from the receiving waters of the Cambridge Gulf to the head of tide. What is the relative importance of tidal flows in sediment transport and deposition compared to river flow in the wet season? The funnel-shaped form of lower Ord, which is typical of many tide-dominated estuaries, required explanation. Measuring the asymmetry of flood versus ebb flows was quite an experience. Results of the process study of the lower Ord, and the more sinuous tidal channel of the King River, were published in the Journal of Geology, 1973, vol. 81, 15-41.

Another component of the two field visits to the Ord and King Rivers was to look at mangrove patterns in relation to landforms and tidal processes. It was a chance for me to build on and test ideas that arose from my earlier ONR/CSI sponsored work in a wet tropical delta region of Tabasco, Mexico (Journal of Ecology, 1967, vol.55, 301-343). The macrotidal nature of the Ord/King areas and adjacent sections of coast presented logistic and floristic challenges; it was exciting to discover some species that others had not found before in this part of the Australian coast.  Our work on mangrove ecology and geomorphology was published in the Journal of Ecology (1975, 63, 203-232).

A later opportunity to work with an ANU team in the South Alligator River in the 1980s highlighted the broader significance of the Holocene evolution of tidal flats as inferred from drilling near the King River. The mangrove fringe of this tidal river was seen as typical of many tidal estuaries in northern Australia. Drilling and dating of sediments rich in mangrove detritus at these different locations from below the tidal or floodplain flats revealed the existence of the “Big Swamp” of around 6000-7000 years ago (Woodroffe, Thom and Chappell, Nature, 1985, vol. 317, 711-713). This was interpreted as a morphodynamic response to continued estuarine sedimentation after sea level stabilised, progressively eliminating intertidal mangrove rich environments. Vertical accretion led to formation of bare high-tidal flats as seen in the lower rainfall Ord/King area, and vegetated plains in the Alligator Rivers region.

While many more recent studies have been undertaken in the Ord estuary, one can only hope that there will be opportunities for others to pursue further studies of estuary process-landform dynamics and evolution in areas where we have had the privilege to work with the support of generous sponsors.

Bruce Thom  

Words by Prof Bruce Thom. Please respect the author's thoughts and reference appropriately: (c) ACS, 2019, for correspondence about this blog post please email austcoastsoc@gmail.com

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