is also more intensive so that the "catchment area" has sufficient capacity to 
supply a creek with sufficient water to scour a neat clear meandering gully. These 
gullies gradually deepen during the rest of the observation period, attracting all 
the water of the later developing backswamp (see below). The contrast in height 
between gully bottom and level of the accretion is also increased by the gradual 
silting up of the area. The vegetation itself favours steep gully walls because of 
its dualistic influence on sedimentation and erosion. In the centre of the stand, 
sedimentation is favoured and erosion hampered. 
The roots also prevent lateral erosion but the edge of the vegetation acts as an 
accelerator of current and increases erosion just outside the edge of the 
vegetation stand (see also ZONNEVELD 1958 and ZONNEVELD 1959(60)). The 
gradual elevation of the soil surface points to the fact that the vegetation 
succession has a mainly allochtonic character. Besides the creek formation, 
the accretion also becomes more differentiated by obtaining a more backswamp- 
like part in the middle and a levee-like character along the sides. The accretion 
is too small for the formation of a large muddy backswamp: only one of a more 
transitional character can be formed. There is also some lateral shift of the 
accretion, with erosion to one side and accretion to the other where another 
moving bar merged with our area (in 1962). On the levee we see a continuation 
of the succession towards a Scirpetum triquetri et maritimi seneoietosum 
and the very beginning of a community of Epilobium hirsutum and Phalaris 
arundinacea. After 1968 this community increased more strongly but then the 
Delta works started to influence the highwater level (lowering of a few dm. on 
average). After 1970 the tidal movement was so much reduced from approximately 
2 meters (+ 130 cm. HW and - 60 cm. LW) to approximately 20 cm. (+ 50 cm. 
HW and + 30 cm. LW), that a totally new environment has been formed, at 
present an explosive development of Epilobium hirsutum, Galium apccrine and 
Urtioa dioica. These plants react on the accelerated decomposition due to 
desiccation and the subsequent liberation of nitrogen, phosphorous, and other 
minerals. Since 1972 a new plant starts to influence the physiognomy on the 
levees: Angelica archangelica, a huge biannual (or triannual) plant, often over 
2 or 3 meters high. Individual plants are clearly visible on the photographs. The 
photos of 1973 show that the new seedlings concentrate around the mother plants. 
It may be expected that in the coming years this plant will dominate the levees. 
Returning to the more central parts of the accretion (with increasing backswamp 
character) we see that from 1950 Typha angustifolia gradually increases, 
joined in 1953 or 1954 by Typha latifolia and Phragmites. These three plants