A Significant Shift in Particulate Organic Matter Characteristics during Flooding of River Krishna, Eastern Peninsular India

Extremely heavy rainfall over a small, semi-arid section of the Indian Peninsula in October 2009, together with release of water from dams resulted in very severe flooding in River Krishna. The sources and type of organic matter during and after the floods were studied by analysing suspended particulate matter (SPM) for organic carbon (C), total nitrogen (N) and isotopic composition of C (δ¹³C_org). The δ¹³C_org varied from –21.4‰ during the initial heavy flood phase to –27.1‰ in the final receding phase. Discharge of terrestrial carbon (–21.4‰ to –23.5‰) from mixed sources with high C/N ratios (14–19) during the initial phase of the flood originated from the semi-arid section of the river. The light carbon (–25.5‰ to –27.1‰) with low C/N ratios (7.2–9.5) in the receding phase of the flood was from local C3-rich organic debris from the deltaic regions along with phytoplankton from aquatic sources. Since the average suspended sediment discharge of River Krishna has decreased from 68 mt to less than 0.1 mt due to construction of dams and barrages, it appears that sediments and organic matter presently being delivered to the oceans are mainly during flood events, and the type of organic matter delivered depends on the nature of the soil where high rainfall is received.