Internet Archaeol. 27. Groot and Kooistra. Flood basin

The flood basins are the lowest areas in the riverine landscape. This makes them especially prone to flooding. Every time a river burst its banks, flood waters brought fertile clay into the flood basins. This explains the nature of flood basin soils: fertile but wet and heavy. Water levels varied between different parts of the flood basins, and throughout the year. The highest water levels occurred during winter. Flood basins are unsuitable for arable agriculture; heavy clay soils are hard to plough with ards and flooding could destroy cereals and other crops.

Although unsuitable for arable agriculture, flood basins offered grazing for cattle, sheep and horses (Fig. 18). Evidence that livestock did actually graze in the flood basins, and not just on the streamridge, is provided by palynological research and mineralised manure found in Kesteren-De Woerd (Fig. 19). In a natural situation, the wet areas of the flood basins are covered in reed and sedge marshes, while the slightly drier parts are covered in mixed deciduous woodland dominated by – depending on the frequency of flooding – willow or alder (Fig. 20; van Beurden 2008). Palynological research has demonstrated that most of the natural woodland had already disappeared from the flood basins (as well as from the streamridges and levees) before the Roman period (Kooistra 1996; 47-50; Kooistra in press; van Haaster 2004). They were replaced by reed marshes and wet grasslands. Wet grasslands often arise from reed marshes that are regularly mown or grazed. When this type of use ends, the reed marshes return. In the settlements of Geldermalsen-Hondsgemet and Kesteren-De Woerd, indications for wet grasslands have been found (Kooistra 1996, 49; Kooistra and van Haaster 2001, 326). During the excavations in Kesteren-De Woerd, mineralised manure from pig or cattle was found. Palynological research on the mineralised manure has proven that the animal's food grew in wet, fertile grasslands (Kooistra and van Haaster 2001, 318-27). The absence of both young lambs and young horses in Passewaaijse Hogeweg in certain phases suggests that sheep and horses were kept in the flood basins during the birth season. This fits in with a system of managing these animals extensively. The rich grasslands also produced hay, essential to feed stabled animals. This was the real asset of the flood basins: as pasture and meadows.

Figure 18: Cattle in a flood basin of the River Narew in Poland. Photo L.I. Kooistra.
Figure 19: Mineralised manure from Kesteren-De Woerd. Photo L.I. Kooistra.
Figure 20: Nature reserve Millingerwaard, adjoining the River Rhine. While not strictly a flood basin, the area is not diked and is therefore influenced by river flooding. Photo M. Groot.

The flood basins and what remained of the riverine woodland also offered a good habitat for wild mammals, although this source of food was only used in a limited way. Some of the remaining (possibly managed) woodland was probably used for gathering wood for fuel and construction.

The total area of the flood basins to the south of the Passewaaij streamridge covered over 400 hectares. This area was surrounded by 12-15 rural settlements, including the two in Tiel-Passewaaij (Ivo Vossen, pers. comm.). Since modern Tiel covers most of the northern flood basins, it is impossible to estimate the number of settlements making use of this area. Flood basins are likely to have been used as common ground by several settlements. During the 2nd century, long ditches are found on the margins of the flood basins. These could reflect a change in perception of ownership of these areas. The ditches were partly dug on an older streamridge located in the flood basin. This section was slightly higher than the surrounding land, and the soil was sandier. Arable farming may have been possible here.

To sum up, flood basins offered grazing for livestock, fodder for stabled livestock, and plant resources (willow, alder and reed) for building (walls and covering roofs), fire and making artefacts such as wicker baskets.

3.1.2 Flood basin