Large estuary systems like the Elbe Estuary usually consist of one main estuary and many tributaries. These tributaries play an important role since they act as transitional waters between marine and fluvial environment. One of the main functions of the tributaries is the drainage of the low-lying marshes in the river catchment. The drainage capability of the tributary depends amongst others on the hydraulically effective cross section, which is influenced by morphological processes. The morphological development mainly depends on the sediment supply from the catchment as well as from the main estuary. In particular, the latter is highly variable due to the non-stationary estuarine turbidity maximum (ETM) in the main estuary (Kappenberg & Fanger 2007). Hence the morphological development of the tributaries − which is for the most parts unknown − can be assumed to be highly variable as well.

In order to analyze the morphological development and its main drivers a field study has been developed and conducted. Within this study the topographies of a tidal tributary of the Elbe has been surveyed in a yearly order for a period of nine years. All measurements were performed with a shipborne multibeam echosounder. A small draft of the research vessel which is only 0.6 m enables the inclusion of the river banks into the survey area. The analysis of the survey data shows that the morphological activity varies along the tributary. The yearly rate of change in bottom elevation varies on average for the whole tributary from −0.15 m up to +0.15 m. Explicit overall trends could not be observed within the considered period since some years show a net deposition and other years a net erosion. However, a correlation could be identified between the morphological development of the tributary and the mean sediment concentration − which may be described as a function of the turbidity − of the main estuary near the junction of the rivers (Fig. 1). A net deposition in the tributary within a period of one year correlates to a high average sediment concentration in the Elbe within the corresponding year. A net erosion in the tributary correlates to a low average sediment concentration in the Elbe, respectively. Figure 1 Correlation between yearly morphological development of the tributary Krückau (km 0−km 7.6) and the corresponding yearly mean sediment concentration (turbidity) in the Elbe near the junction of the two rivers (<xref ref-type="bibr" rid="ref126_2">Nehlsen 2016</xref>). https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315623207/4fbc492d-6678-4a12-aaf6-5c2b8ea38e5f/content/fig126_1.tif"/>

This finding indicates that the turbidity and the position of the turbidity maximum of the Elbe estuary is an important driver of the morphological development of tidal tributaries. However, the mean variation of points indicates that the sediment supply from the main estuary may not be the only relevant driver. Further research should identify other important drivers, including the sediment supply from the catchment, in order to develop elaborated models, that are abled predict the long-term morphological development of the tributaries.