River deltas

In courses of running water there is a continuous transport and re-deposition of particles detached by weathering and erosion. The finer-grained particles (fine sand, silt and clay) are transported in suspension, while the coarse sand and heavier particles are mainly carried along the bottom. If the current velocity decreases, the transport capacity will also decrease and material will be deposited. At river mouths there is usually a rapid decrease in the current velocity and the sedimentation which takes place at these points often results in the formation of a river delta.

Old view from Mount Tjakkeli in northern Sweden looking along the delta front in Lake Laitaure. August 19, 1954. Lateral drainage channels, formed by meltwater during the deglaciation period, are visible along the mountain slope in the background. Below is the homestead of Aktse.

The extensive delta formations which arise at river mouths in lakes and seas are an obvious testimony to the eroding, transporting and accumulating action of the rivers and rapidly reflect changes in the activity of the fluviatile, marine and/or lacustrine processes. Continual splitting, formation, oscillation and closing of distributaries characterize the active phase of delta formation, by which the normally branching delta pattern is created and modified.

The first part of my doctoral thesis (Axelsson, V., 1967: The Laitaure Delta. A study of deltaic morphology and processes. Geogr. Ann. 49 A) deals with general conditions for the formation of river deltas, the flow pattern at the mouths of deltaic channels, some morphological features of the delta front and the delta plain, and with the formation, splitting and closing of distributaries.

Schematic formation of deltaic distributaries and inter-levee basins. From Axelsson 1955 (Ymer 75). For explanation and discussion, see the link "formation".

The first part of the thesis also deals with delta structure and with the sedimentary characteristics of delta plain and delta front deposits.

The morphological development of a branching river pattern is greatly dependent on the special conditions for erosion, transportation and deposition of sediment at the head of channel forks. These processes, that are of fundamental importance for the development of deltas, are further discussed in the second part of the thesis. This part also presents the results of experimental studies on the diversion of sediment at branching channels.

The third and final part is devoted to an account of hydrological, morphological and sedimentological data of the Laitaure delta. It is situated at the eastern border of the Scandinavian mountain range, just north of the Arctic Circle, and has been formed in Lake Laitaure by the river Rapaälven, which drains the central part of Sarek, the most glacierized high mountain region of Sweden. Half of the total annual discharge of sediment to this delta takes place, on an average, during a total period of only 5 days. This means that some years half of the total annual delta growth may take place during a single week.

The central part of the Laitaure delta in northern Sweden with active and abandoned river channels, bordered by natural levees, which surround different types of inter-levee basins.

The Laitaure delta is an area of subdued relief that sharply contrasts with the adjoining mountains which face their bare rock walls and steep talus slopes to the delta. Active and abandoned channels, levees and inter-levee basins constitute the major elements of the delta plain and surround islands and peninsulas of the former lake basin, as partly shown by the map below.

The proximal part of an old map of the Laitaure delta.

This map is mainly based on air photographs taken on June 18, 1954. The map was originally drawn on a scale of 1:4,000. This original map has a mean error of about 2 m in plan (sharp details) and 1 m in height (open terrain). For the sake of simplicity the contour lines are given in metres above the water stage that prevailed in Lake Laitaure when the above mentioned air photographs were taken.

A new map of the Laitaure delta, based on air photos taken on September 4, 1990, was published by Andrén in 1994 (UNGI Rapport Nr 88) and used for analysis of the development of the delta since 1954.

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