Mike Blum
University of Kansas, Lawrence, United States of America
Mike Blum received his Ph.D (1992) from the University of Texas at Austin, focusing on climatology, geomorphology, and sedimentology. He held faculty positions at Southern Illinois University (Assistant Professor 1991–1995), the University of Nebraska-Lincoln (Assistant and Associate Professor, 1995–2003), and Louisiana State University (Harrison Family Professor, 2003–2008), then served as Research Advisor at Exxonmobil Upstream Research (2008–2014). His research interests include fluvial to shallow-marine processes and deposits, connections between fluvial-deltaic and deepwater systems, and source-to-sink analysis. Mike is now the Ritchie Distinguished Professor in the Department of Geology of the University of Kansas, where he pursues these themes through his research and teaching.
On a global scale, major delta systems aggraded and prograded during the middle to late Holocene when global sea-level rise slowed to rates of < 1mm/yr. However, most major delta systems face considerable uncertainty in the... [ view full abstract ]
On a global scale, major delta systems aggraded and prograded during the middle to late Holocene when global sea-level rise slowed to rates of < 1mm/yr. However, most major delta systems face considerable uncertainty in the near future due to human activities that have changed inputs of water and sediment, and accelerated rates of global sea-level rise. The Mississippi delta is one such example, and is now in the midst of a profound transformation, where >25% of deltaic wetlands have disappeared during the last 100 yrs, and the region is increasingly vulnerable to storm surge. Land-building diversions of Mississippi River water and sediment are envisioned to achieve sustainability of delta surface area, but present scientific, political, economic, and cultural challenges. This presentation examines technical issues that link future sea-level rise and sediment mass balance, and uses the Mississippi as a case study to address sustainability of deltaic landscapes.
The Holocene Mississippi delta reflects filling of a glacial-period incised valley followed by progradation and construction of an extensive delta-plain topset. Long-term rates of deposition required to fill the valley and construct the delta plain over the past 12,000 yrs are ~230 megatons of sediment per year (Mt/yr), with remaining supply dispersed to the shelf. Prior to the 20th century, >400 Mt/yr of sediment were delivered to the delta plain from a drainage basin of >3,400,000 km2, then dispersed through crevasse and distributary channel networks to a delta topset that exceeded 20,000 km2. However, >40,000 dams within the drainage basin now trap ~50% of the natural sediment load, whereas construction of continuous levees have decoupled the delta-plain topset from fluvial sediment input. Hence the delta plain is now both supply- and transport-limited. Moreover, rates of global sea-level rise are now >3 times faster than they were during construction of the delta-plain topset. The unintended but unfortunate convergence of engineering activities with accelerated sea-level rise resulted in the now well-documented rapid 20th century submergence of the Holocene delta plain.
Future delta landscapes cannot resemble the recent past. For the Mississippi case, the present transport-limited condition can be mitigated by land-building and land-sustaining diversions that reintroduce river sediment to delta-plain wetlands. However, the present supply-limited condition ensures insufficient mineral sediment to sustain a large portion of the delta-plain surface. Rrates of global sea-level rise are currently ~3 mm/yr, and expected to accelerate to >4 mm/yr by 2100. Without diversion of sediment to the delta plain, a conservative sea-level rise and subsidence scenario will inundate the ~10,500 sq. km that is now <0.5 m in elevation by the year 2100, whereas a worst-case scenario will inundate >13,500 sq. km. ~18-24 billion tons of sediment will be required to sustain surface area to the year 2100, more than can be delivered from the Mississippi drainage basin in the current supply-limited state. However, even if natural loads were restored, global sea-level rise is >3 times faster than during the 6000 yr period over which the delta plain was constructed, and significant drowning is inevitable.
