Upper catchment floodplain aggradation and channel incision processes - abstract to a thesis chapter

So, yes, that title is officially: boring as shit. I can't help it. If I call it "how creeks fail epically" I get laughed at. But my boring title starts to really mean something when I take the abstraction away and personify the stream -- how do streams build floodplains, what causes them to stop doing that and then cut into and wash away all that effort in a hundredth of the time it took to build. Very often we can't blame the creek for stuffing up as its how we've managed the land that is the cause.

Yet that doesn't say why we care. If the stream erodes a bloody great channel through the floodplain then we change how that system works. The main thing that changes is how the water moves through that land. It now buggers off down the channel and doesn't hang around like it did before. Since accidently killing a potted cactus, I've scientifically determined that plants tend stay alive better when they have water. So if all our creeks are leaking the water that they used to hold onto it makes things more difficult if we want to grow stock fodder, revegetate the creek banks or keep the helpful critters in the water happy.

Below is the translated version of my thesis chapter summary. Below that is (the sixth re-incarnation of) the original abstract if you wish. Though I have replaced all my references with the word REFERENCE/S to spare you a list of context-irrelevant names and years.



Chapter Summary

Where we haven't accidently stuffed the land, the creeks and surrounding floodplains have lots of variations. Although they may look different, they all have a large amount of dirt (from road gravel and beach sand size to "I swear I'm not making phalluses with the modelling clay" clay size) as a floodplain that they have built up over hundreds to thousands of years. They also hold onto a lot of water in the floodplain, far more than we see in the creek at any one time. The water held there slowly soaks out into the stream keeping it flowing when its dry.

Where we have accidently stuffed the land we see the creek chomp down and out into the floodplain making a (usually) deep channel. The channel lets much of the water that was kept in the floodplain leak out and the dirt gets washed downstream along with it. Now that we have the deep channel in the floodplain the landscape can't hold onto water like it did. So overall, the water doesn't stay in the floodplain very well, when it rains the water sprints off down the channel too quickly to be much use, and when its dry the creek doesn't flow much if at all. The creek is a bit crap basically.

This chapter of my thesis takes what everyone else has said about creeks (both when they work and when they are stuffed), puts it all together and attempts to make me look good by exclaiming "now isn't that interesting" when really everyone already knows its interesting. Also, of all the creeks in all the world I'm looking at the ones in and around Canberra because no-one would give me the money to go and look at the creeks in Hawaii, or the French Riviera, or...



Full Abstract

Lower order drainages (identified here as upper catchments) and associated floodplains broach a wide range of landscapes and morphologies, ranging from swampy meadows and chain-of-ponds, to shallow channelled streams encompassing the full gamut of fluvial features. Irrespective of the floodplain and drainage morphology, the common features of these catchments are a significant alluvial sediment deposition and a perched alluvial aquifer that may or may not be hydrologically connected to the broader groundwater system (REFERENCE/S).

Channel incision into the floodplain sediments of upper catchment drainages is a common global phenomenon (e.g. REFERENCE/S). Regardless of whether it is the result of natural landscape evolution (REFERENCE/S), land management induced erosion (REFERENCE/S) or deliberate drainage channelisation (REFERENCE/S) the result is a profound impact on the hydrogeomorphic function of these landsystems. Incision typically involves significant sediment mobilisation and transport to lower reaches, and drainage of the alluvial aquifer affecting the surface water-groundwater (SW-GW) interactions, particularly groundwater residence times and storage (REFERENCE/S). As is expected, these changes have a strong alteration of the hydrogeomorphic character of upper catchments.

This paper focuses on the unique south-east Australian context of upper catchments and channel incision and provides a review of the hydrogeomorphic processes in intact and incised upper catchment floodplains and the changes that occur in that transition.