The river Rapaälven

The river Rapaälven, which drains the central part of the high mountain region of Sarek, has a total length of 74 km, and a length of 53 km upstream of the Laitaure delta. The downstream part of Rapaälven (downstream of lake Laitaure) is also called Laitätno. According to Andrén the drainage area comprises 662 km2 at the Litnok hydro-meteorograph, and 677 km2 at the proximal part of the Laitaure delta. This drainage basin is roadless, the larger part highly inaccessible, and comprises the most glacierized region of Sweden. Lakes constitute only 0.3 % but glaciers as much as 11 % of the drainage area upstream of the Laitaure delta.

The hydro-meteorograph hut at Litnok

In order to obtain continuous data of the water stage in the river Rapaälven and also of certain meteorological elements (temperature, humidity, wind and precipitation) Axel Hamberg mounted a specially designed hydro-meteorograph at a fast flowing part of the river, just below the upper rapids at Litnok, 3 km upstream of the Laitaure delta. This instrument was brought into use in the summer of 1915.

 

Mean and extreme water discharge in Rapaälven at Litnok during the summer half-year, determined from mean daily flows (1916-1945). From Axelsson 1967 (Geogr. Ann. 49A).

As shown by the diagram the water discharge may be very high or rather low on every day in summer. The water discharge normally begins to increase more markedly in May when the snow begins to melt. The production of melt-water from the snow cover and from the glaciers, however, is often interrupted by cold weather. Periods with heavy rain are normally very short. The steep topography, the small thickness of the soil cover, and the lack of larger lakes favour a rapid run-off. The frequency of individual floods is therefore very high from May to September in relation to the size of the drainage basin, while the duration of the individual floods is very short.

Flood-frequency curve for Rapaälven at Litnok, determined from annual floods (1916 - 1945).

The flood-frequency curve is based on only the maximum flood peak of each calender year 1916 - 1945. The mean value of the annual-flood series is 294 m3/sec. The standard deviation is relatively small and amounts to only 62 m3/sec. The 30-yr flood, for example, is therefore only 1.5 - 1.6 times higher than the mean annual flood. The bankfull discharge in the proximal delta section (about 280 m3/sec.) has a recurrence interval of 1.5 - 2 years according to the annual-flood series (about 1.4 years according to the partial-duration series). This recurrence interval of the bankfull discharge is about the same as found for flood plains in general.

The annual variation of the water level is normally of the order of 2.7 m at Litnok. The water level rises and falls very rapidly in summer with changes in precipitation and temperature. The rates of both rise and fall may exceed 1 m in 24 hours. During dry periods in summer with a marked daily variation in air temperature, there is also a very distinct daily variation in water stage.

The water level is normally dammed up by the ice in the early part of the winter. This ice-damming usually begins in October with the undercooling of water and the formation of frazil and anchor ice. When the rapids are also covered by ice the water level normally falls, leaving the ice as a bridge across the stream, supported by boulders, and in late winter the water level is usually not influenced by the ice.

Water stage and air temperature at Litnok during April 23 - 27, 1958.

In April and in the beginning of May, however, a diurnal type of ice-damming often occurs when parts of the rapids at Litnok are ice-free, the diurnal range in air temperature is great, and the nights are very cold. An example of such a situation is illustrated above.

Suspended-sediment discharge rating curve for the river Rapaälven at the proximal part of the Laitaure delta.

At this river station the calculated discharge of suspended sediment amounted, on an average, to 160,000 metric tonnes/year. The annual variation in suspended sediment discharge is extremely great. The mean annual extreme values are lower than 0.01 tonnes/day and higher than 20,000 tonnes/day respectively. About 95 % of the total annual suspended-sediment load is discharged during the three summer months June - August, and half of it during a total period of about 5 days. This means that some years half of the total annual growth of the Laitaure delta may take place during a single week.

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