The role of flood size and duration on streamflow and riparian groundwater composition in a semi-arid basin

Floods with differing sizes and durations are likely to impact riparian systems in different hydrologic and geochemical ways. Here the impact of flood size and duration was investigated. Flood-driven recharge along predominantly losing reaches of the Bill Williams River (western Arizona, USA) later reemerges as baseflow in downstream gaining reaches, and the river's longest losing reach (Planet Valley) retains and releases the most flood recharge. River discharge volume and flow intermittency downstream of Planet Valley is highly dependent upon the length of time since the last major flood. After large floods (e.g. 2004-2005), baseflow was dominated by the flood's chemical (SO₄, Cl) and isotopic (d¹⁸O_H2O,δ²H_H2O,δ³⁴-S_SO4,d18O_SO4) composition for long periods (>4 years), suggesting that the largest events result in much more flood recharge and a longer persistence of floodwater in the subsurface than after smaller, more recent events. The continued dominance of baseflow by 2004-2005 floodwater nearly 5 years later- despite three smaller floods in 2007, 2008 and 2009-highlights the long-term impacts that the largest floods have on riparian water composition. Of these three recent events, only the largest and longest (in 2009) caused observable changes in both baseflow volume and the composition of baseflow and riparian groundwater, thereby suggesting that a threshold of flood size and duration exists for floods to alter the system's state and behavior. The river's dependence on large winter floods and the tendency of a particular set of atmospheric conditions (associated with El Niño-Southern Oscillation, or ENSO) to cause the region's largest winter floods indicates the importance of ENSO to the system, and that future changes to ENSO caused by climate change could drastically alter the flood properties and overall hydrology of southwestern rivers.