SEDIMENTARY PROCESSES IN AN EPHEMERAL RIVER AND TERMINAL SPLAY COMPLEX, LAKE EYRE, CENTRAL AUSTRALIA – OBSERVATION FROM THE FEBRUARY 2003 FLOODING EVENT

Tobias H.D. Payenberg¹, Mark R.W. Reilly¹, Simon C. Lang¹, and Jochen Kassan^1,2
¹ Australian School of Petroleum (ASP) and Australian Petroleum Cooperative Research
Centre (APCRC), University of Adelaide, Adelaide, SA 5005, Australia
² Whistler Research Pty Ltd, Whistler Court, Greenbank, QLD 4124, Australia

Ancient ephemeral fluvial systems are of increasing interest as reservoirs. Analogues from comparable modern systems can therefore play a useful role in understanding depositional processes that control facies distribution, geometry and connectivity. A key aspect of these environments is the role of variable discharge flood events, and floodwater and sediment type distribution within the drainage network. Observing these flood events is a rare opportunity because of their short duration, and typically remote location. This study outlines the results of aerial photography and follow-up fieldwork of a typical flood event in central Australia leading into Lake Eyre, a large ephemeral playa lake.

In the week of February 16-22, 2003, an offshoot of a cyclone crossed central Australia from NNW to SSE, passing just to the west of Lake Eyre itself. This cyclone brought over 200mm of rain within five days of the week ending February 23, 2003, falling after a dry period of about 2 years. This rainfall event provided the rare opportunity to document the hydraulic and sedimentary processes active within the rivers, crevasse splays and terminal splays that comprise the Neales and the Umbum rivers and associated terminal splays that flow into the western side of Lake Eyre.

The Neales and Umbum Rivers both comprise channel segments that fall under different river classifications. River segments that show an anastomosing and braided channel appearance are called anabranching river segments. This classification refers to the bars within the river that are stable and vegetated, unlike a typical braided river system where the bars are highly mobile. Channel segments that are more straight or meandering also exist. For the most part of their course, the Neales and the Umbum Rivers are currently cutting down into the substrate, and are thus erosional, not depositional. Anabranching channel segments are typically found in areas where the channel margins are far apart, and the river has the space to expand and anabranch, stabilised by vegetation that grows within the channels. The straighter and meandering segments are typically found where the rivers are laterally restricted in their flow, thus forming a deeper, single channel, incised along most segments.

Floodwaters within the ephemeral streams of the Neales and Umbum drainage system typically move quite independently from one another. Rainfall within the region is often extremely patchy resulting in localised downpours and the flowing of different tributaries and river segments at different times. This can ultimately lead to the scenario where a heavily drenched area is draining into local creeks that flow into the river that is dry at the time the water actually reaches it.

Only if sufficient water has collected in the river system and after the soil and ground is sufficiently waterlogged, can the river actually begin to flow. In the Umbum and Neales Rivers, actual flash flooding is rare, though anecdotal records exist. Typically the flood front moves at a speed of metres per hour. The process by which the channel fill is that of pooling of water within the deeper parts of the river and a “spilling” effect into the next pool downstream, thereby slowly filling the channel and enabling a flow.

Once enough water pools in the channels and sufficient rain has fallen to form a continuous flow for some time, the water masses are channelled through the distributary channels on the respective terminal splays of the Umbum and Neales Rivers.

Prior to the flooding event of February 16-23, 2003, Lake Eyre was dry, and had not received any significant water since mid 2000. Water levels in Lake Eyre are predominantly controlled by the larger river systems draining into the north east of the Lake, such as the Diamantina River and Cooper Creek, which drain a large part of New South Wales and Queensland (1,140,000 km²). The drainage areas of Umbum and Neales Rivers are comparatively small (37,000 km²), and hence the waters these rivers bring into Lake Eyre are of lesser significance. Lake Level is therefore largely independent of water discharge in the Neales and Umbum Rivers. For that reason, the terminal systems of the Umbum and Neales Rivers sometimes act as a delta (during high lake levels) or as a terminal splay system (during low lake levels).

During the flooding event of February 16-23, 2003, lake levels were low, so that the modern Umbum and Neales terminal systems can be classified as terminal splays. Floodwaters dispersing over the splay-plain reached the frontal splay complex and then flowed as a thin veneer of water over the distal splay area before seeping into the playa lake bed.

The morphologies at the Neales and Umbum terminal splays are slightly different, as the Neales Delta is a larger, more mature system, and therefore they will be discussed separately. The source areas for the two systems are also different, resulting in the Umbum system being significantly coarser grained compared to the more mixed sediment composition of the Neales.

Neales Terminal Splay Complex

The distributary channels on the Neales terminal splay complex filled with water until a bank-full flow was reached. At this point, the water spilled into the channels of the crevasse splays and terminal splays, which have a shallower channel depth (<2m) compared to the distributary channels (<5m). In the southern distributary on the Neales terminal splay complex, the crevasse channels are branching off in a northerly direction, and there are several channels that branch off every few 100s of metres (Figure 1).

Beyond the distributary channel, water flows not only within the crevasse channels, but also as a sheet of water across the landscape. On aerial photographs, the crevasse channels stand out clearly as a vegetated channel system, but this is due to the fact that more moisture is preserved within them (Figure 1). When the water leaves a channelised flow, the water disperses rapidly into a sheet flow, during which the velocity rapidly declines and fine-grained sediment settles out of suspension. This process of splay lobe (middle ground bar) formation is similar to the construction of a mouthbar in a deltaic system. With maturing age of a splay system, the area appears like a braided system of sandy channels and sandy/silty middle ground bars. This results in a variable net to gross distribution across the Neales terminal splay.

Umbum Terminal Splay Complex

The Umbum Creek terminal splay differs from the Neales in that it comprises only one large, main channel system that ends in a terminal splay complex. There are no branching distributary channels on the Umbum splay (Figure 2). However, once the main channel reaches the playa margin, and the water disperses at the river mouth, a complex network of splays with minor channels emerges (Figure 2). The size of the individual splay lobes at Umbum Creek is much larger than those on the Neales. This is due to the Umbum Creek comprising only one main, deep fluvial channel, approximately 120m across at the mouth, compared to the many smaller distributary channels of the Neales, which are only about 50m across at their mouths. The increased channelised discharge of the Umbum Creek leads to the larger size. The splay lobes form in the same way as described for the Neales, but the system is overall shallower and broader.

The significance of these observations is that the variable discharge flood events, and the differing sediment yield (coarse-grained sands in the Umbum versus mixed sandy and muddy sediments in the Neales) results in contrasting styles of terminal splays along depositional strike. Because the two systems are only 50km apart, the range of variability observed can be expected in subsurface reservoirs of similar depositional setting.

Figure 1: Aerial photograph of the bank-full, southern Neales terminal splay distributary.