Hydro-ecological relations in the Delta Waters

The storm surke of L&Z1 A.D., knorm as St. ELisabeth's flood, had a
big impact in this respect. It caused a drastic change in the courses
of the =in rivers and conaiderable losses of laud (Rig. 1). Due to
these land losses - the extensive tidal marsh lands of the Biesllosch
originate from this flood - the tidal prisms of the early Haringvliet.
erevelingeu and Oosrerschelde increased considerably, resulting in a
further deepeniag and widening of these basins. The former Eeliniua on
the contrary showed a decrease of tidal prism and as a result silted
up. This pzocesa of siltation still was continuing during the 19th
century, lead* to the need of coustmcting the Botterdam Waremay
(1868) fm order to ensure aecess to the Botterdam harbours (Van den
Berg. 1987).
The Qosterschelde inLet hardly did not change position since Roman
times. Bowever, the shape oE tbe estuary ewersince has changed
considPrably. Originally the Oosterschelde was the main distributary of;
the river Skheldt. From about 700 &.D. the mere southerly Westerschelde
estuary developed from a small tidal inlet, during Rman times located
approximately at the present Dutch-Belgian border (Rig. 1; benders.
19861. The Westet~chelde gradually took over the discharge of the river
Gcheldt. At the end of the 16th century the Oosterschelde practically
had lost all importance as a distributary of Scheldt waters.
Revertheless, in contrast to the silting up of the Heliniua after
diparsim of the mHin courses of the rivers Rhine and Pleuse, the
Oostersmhelde showed intensified erosion. An explanation for this
renarkable different behaviour Q the dramatie increase of the
Oostersehelde's tidal prism due to extensive lwd losses. Apart from
the effeers of the earlier mentioned St. Wsabeth's flood. it 1s
estimated that as a result of another notorious storm surge
(St. Pelix's flood of 1530). the Ooseerschelde basin was enlarsd to
such extend that it's tidal prism must have increased by at least 50%
(Van den Berg, 1986). The erosive respouse to this inczease of tidal
prism still is visible during the 19th century CFig. 2).