Development of the Laitaure Delta, Swedish Lappland A Study of Growth, Distributary Forms and Processes

Abstract: Deltaic processes were studied on the Laitaure delta. Specific aims were to: analyse forms and processes in distributaries with regard to hydraulic geometry relationships; investigate the distribution of water into different distributaries; analyse the morphological development of the delta; and quantify the advance of the delta front. Further, a brief study was made of the bottom sediments in Lake Laitaure.It was concluded that distributaries may be distinguished by at-a-station hydraulic geometry relationships, depending on their state of activity and their location in the delta, i.e. distance to the delta front.Differences in activity between distributaries were also shown by the “delta” (analogous to downstream) hydraulic geometry relationships. Large similarities in “delta” hydraulic geometry were found between the Laitaure delta and the Volga and Danube deltas (cf. Mikhailov, 1970).It was found that the discharge in a distributary is linearly related to the total discharge in Rapaälven, and that the main distributary system may, inter alia, be distinguished from the other systems by its: (1) higher rate of increase in discharge with total discharge;  (2) decreasing proportion of the total discharge as this increases. Threshold discharges, i.e. the total water discharge in Rapaälven below which a channel has zero discharge, were determined for the “secondary” distributaries.The temporal variation in flow distribution is indicated by, for example, changes in the rate of deltaic advance at different mouths and aggradation of channel beds within distributary systems that have experienced reduced flows. It is concluded that the flow distribution is an important factor to be considered in studies of river deltas.The delta is growing by about 0.38 km2/year, equivalent to an advance of the delta front by approx. 4.5 m/year. Disintegration of vegetation is abundant all over the delta except for the distal parts. However, the proportion of delta lake area has decreased and the proportion of vegetated area has increased due to miniature delta formation, and deltaic growth.X-ray radiographs of sediment cores sampled in different parts of Laitaure, reveal annually laminated deposits, for which the variations in temporal and spatial sedimentation rates were investigated.