Man made deltas

Abstract

The review of geochronological and historical data documents that the largest southern European deltas formed almost synchronously during two short intervals of enhanced anthropic pressure on landscapes, respectively during the Roman Empire and the Little Ice Age. These growth phases, that occurred under contrasting climatic regimes, were both followed by generalized delta retreat, driven by two markedly different reasons: after the Romans, the fall of the population and new afforestation let soil erosion in river catchments return to natural background levels; since the industrial revolution, instead, flow regulation through river dams overkill a still increasing sediment production in catchment basins. In this second case, furthermore, the effect of a reduced sediment flux to the coasts is amplified by the sinking of modern deltas, due to land subsidence and sea level rise, that hampers delta outbuilding and increases the vulnerability of coastal zone to marine erosion and flooding.

Figure 1. Location of the four delta systems with catchments and growth phases.

Drainage basins of the largest southern European river systems on SRTM 90 m Digital Elevation Data (CGIAR-CSI), with major dams and reservoirs (red dots) and location of the Soppensee Lake (black star; see pollen spectra in Figure 3). The inset graph shows the number of river dams built per year since 1900 AD; the temporary decline in dam construction in the '40 can be attributed to the Second World War (see Supplementary S5). Below: satellite views of modern deltas (from the Landsat Archive and the Global Land Survey), with red lines representing their embryonic stages, during the last phases of post-glacial sea level rise (between 9 and 5.5 kyr BP), and the white lines representing the main dated progradational events of delta growth (see Supplementary S3).

Figure 2. Correlation of linear progradation rates of the four delta systems.

Summary of the main intervals of delta growth of the Ebro, Rhone, Po and Danube systems, reporting their maximum rates as orders of magnitude (see Supplementary S1 and S3). Note that the time window is in two scales with 1 kyr steps, before 1500 AD, and 20 yr steps, onwards; delta growth rates are also summarized against a linear time scale in Figure 3. Two major and substantially synchronous phases of delta construction correspond to the expansion from 300 yr BC to 250 yr AD, and the last ca. four centuries, matching the Little Ice Age. Retreat of major delta fronts occurred after the Roman Empire and impacted almost all delta lobes during the last 50 yr (red bars).

Figure 3. Multi-proxy reconstructions of climate change, human diffusion and delta progradation.

The last 3 millennia were characterized by several climatic oscillations, synchronous at northern Hemisphere scale, with two major extremes in the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). The trend of pollen record from the central Alps reflects temperature and precipitation changes all over the Europe region and can be compared with the temperature record derived from Greenland ice core. Growth rates of the main four northern Mediterranean and Black Sea river deltas (see Supplementary S1, each step is one order of magnitude) increased during the Roman Empire, and dramatically since the onset of the Little Ice Age (LIA), characterized by closely-spaced intervals of advancing and waning of Alpine Glaciers, increased flood frequency, high storm activity, but only a slight increase of total precipitation. These natural factors may reflect the occurrence of repeated and closely spaced NAO-index anomalies (, positive, red, and negative, blue) and several centuries of overall decreasing T. While human expanded, in the past, during intervals of milder climate (Roman Empire), this was the first substantially cooler interval severely impacted by humans through, inter alia, forest clearance, increasing population and related land consumption. The change in land use is well reflected by increased shrub+herb vs. tree in pollen spectra both in the alpine region and in the Mediterranean, especially in the second half of the LIA.

Figure 4. Modern water and sediment flux.

Coupled diagram of suspend sediment load (brown) and fresh water (light blue) demonstrating the impact of anthropic reservoirs on river discharge in all four Mediterranean deltas; gray stripes highlight the timing of construction of major dams (Supplementary S3 and S5). Peaks of water discharge are in phase with peaks in suspended sediment load only before dam's construction until the '60.