Floodplain farmland is so fertile because rivers flood

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Recent concerns have been expressed by Murray farmers downstream of Lake Hume about the devastation that flooding may cause to their properties. The prospect of more frequent flooding because of environmental flow releases, as part of the Murray-Darling Basin Plan, has got them worried.  While this is understandable, and flooding can cause a lot of damage to farm infrastructure, productive farmland on floodplains only exists because of the flooding – and we should not forget that.

Virtually by definition, rivers flood about every two years.  The reason for this is because of the relationship between geology, geomorphology and climate. When they flood, water moves out on to the floodplain (of course).  But so does sediment and a lot of organic matter – one of the major sources of carbon (and so energy) and nutrients – which drives floodplain processes. There is, of course, interchange between the river and floodplain and river again. Scientists often ask questions like: “Are floodplains sources (from floodplain to river) or sinks (from river to floodplain) of organic matter and nutrients?”. Well, they are both. But they are certainly at times major sinks, which is what makes the land so fertile.

Researchers working in large rivers in Europe have estimated that flooding can deposit 250 tonnes per hectare per year (t/ha/yr) of sediment, 96 t/ha/yr of fine particulate organic matter (really small bits of bark, leaves etc), 2.9 t/ha/yr of particulate organic carbon (suspended organic carbon too large to pass through a filter) and 0.96 t/ha/yr of nitrate nitrogen (Tockner et al. 1999). Another study has estimated that 80% of particulate organic carbon on floodplains can come from rivers (Hein et al. 2003). Studies in other types of rivers, even glacial and upland rivers, have found that organic matter gets deposited in quantity on floodplains during flooding. This will all contribute to the fertility of floodplain soils and make them ideal places to grow crops or graze cattle and sheep.

Flooding also has a major influence on the organic matter that may have built up on the floodplain from terrestrial sources, like trees, shrubs and grasses. Studies have shown that by flooding organic material, especially leaves, they release large amounts of dissolved organic carbon (O’Connell et al. 2000). Flooding enhances the breakdown of organic matter on floodplains generally and much of this becomes available for use by animals and plants. This carbon may be important for the aquatic system, but when floodplains dry out again, this carbon can be important for the newly formed terrestrial system.

Like in Australia, floodplains are farmed in most parts of the world, because the soils are so fertile. In Peru, for example, subsistence farmers preferentially farm the edges of rivers because they are the best soils, and because there are other ecosystems services – like pollination – provided by riverside vegetation (McClain and Cossio 2003). Of course, the well-known annual flooding of the Nile, and subsequent rejuvenation of the land ,was the source of much of the wealth of ancient Egypt – which sustained it for millenia (Sparks, 1995). Hughes (1992) and Butzer (1976) before him, believe that is impossible to think of ancient Egypt without considering the ecology of the River Nile. They quote Herodotus and Pliny the Elder as describing the Nile as “the farmer” and that Egypt was “the gift of the Nile” (Hughes, 1992).

Perhaps that is the way that we should look at the floodplain farming land along our rivers: that it is the gift of the river which runs alongside them.

References

Butzer, K.L. (1976), Early Hydraulic Civilization in Egypt (Chicago: University of Chicago. Press). Hein, T., C. Baranyi, et al. (2003). “Allochthonous and autochthonous particulate organic matter in floodplains of the River Danube: the importance of hydrological connectivity.” Freshwater Biology 48(2): 220-232. Hughes, J. D. (1992). “Sustainable agriculture in ancient Egypt.” Agricultural History: 12-22. McCLAIN, M. E. and R. E. Cossio (2003). “The use of riparian environments in the rural Peruvian Amazon.” Environmental Conservation 30(3): 242-248. O’connell, M., D. Baldwin, et al. (2008). “Release and bioavailability of dissolved organic matter from floodplain litter: influence of origin and oxygen levels.” Freshwater Biology 45(3): 333-342. Sparks, R. E. (1995). “Need for ecosystem management of large rivers and their floodplains.” BioScience: 168-182. Tockner, K., D. Pennetzdorfer, et al. (1999). “Hydrological connectivity, and the exchange of organic matter and nutrients in a dynamic river–floodplain system (Danube, Austria).” Freshwater Biology 41(3): 521-535.

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