Channel morphology and changes between 1957 and 1983 in Reach B. Numbers refer to locations shown in Figure 1B 

Contexts in source publication

Context 1

... and floodplain morphology (1988±1997) Reach A. The morphology of the river and floodplain of this reach is generally characterized by low gradients (`0Á0001) and a type of anastomosing. Anastomosing is here defined as a river that is divided into totally separate anabranches over distances greater than one meander wavelength. It is synonymous with braiding except that in this case the anabranches are not located in the same channel; further detail on types of anabranching and anastomosing can be found in Knighton (1998). Two distinct anabranches exist. The western one is currently dominant and exhibits a meandering planform along its 14Á7 km length (Figure 3). Adjacent to this western anabranch infilled meander cutoffs are apparent. The more easterly anabranch is much smaller and flows over the floodplain devoid of features demonstrating migration by the Luangwa River except at its northern end. Here the start of the anabranch follows the line of a former meander of the Luangwa River prior to cutoff at some time in the past (pre-1956) and forms a big loop. To the south the anabranch has occupied the line of the much smaller Chibembe tributary. Between the two anabranches 2 km upstream of their merging, a shoaled abandoned channel is also apparent (location 2, Figure 3). Reach B. Reach B is characterized by a single-thread meandering channel. To the north of the reach it currently exhibits a low sinuosity meandering river but tortuous asymmetric meanders are apparent in the south. In this lower section the floodplain is a series of old infilled meanders and cutoff lagoons of recent origin which show that the river in the recent past was highly tortuous in planform (Figures 4 and ...

Context 2

... of channel planform instability. The Luangwa River in the last 50 years has exhibited different types of channel change as described below. I. Avulsions. Avulsions were observed to occur generally at bends, forming as a result of water superelevation on the outside of banks during flood conditions, and these areas are the first point of overspilling onto the floodplain (P. Berry, personal communication). There is a tendency for avulsions to exploit minor channels, or old courses of the Luangwa River, which enter the main river immediately upstream or downstream of a meander apex where floodplain elevation is lower. The reason for the existence of minor channels near the bend apex is not known. An example of an avulsion which exploited a minor channel is the cutoff at the Luangwa±Kauluzi River junction (bend 11, Figure 7). This most likely occurred in 1971 when one of the highest floods was experienced in the area; in 1967 flow was still around the meander yet by 1974 flow was principally through the avulsion. Another avulsion occurred in the early 1980s when the Luangwa River changed from being single thread to having two main channels, the newly formed 16 km channel for much of its course reoccupying a `relic course' of the river (reach A, Figure 3). The consequence of avulsion and meander cutoff is channel straightening. This is well demonstrated in reach D (bends 18±20), where in 1956 three meanders with a planform indicative of lobing were present but by 1967 had been replaced by a fairly straight channel with large alternating bars. In 1956 signs of overbank floodwaters spilling across one meander are visible but the exact time of avulsion is not known. Three distinct former meander bends, partially infilled and subject to compete drying up in summer, are now present on the floodplain (Figure 8). Meander cutoff by avulsion rather than the gooseneck being eroded completely by bank erosion appears to be the norm given the morphology of palaeomeanders. The large number of palaeofeatures visible in the field and on the aerial Earth Surf. Process. Landforms 25, 421±436 (2000) photography is indicative of how frequently avulsions have taken place in the past; at least seven have occurred in the last 40 years. The presence of high bank erosion rates and the large number of palaeomeanders combined with low floodplain elevation provide frequent opportunities for the active channel to abutt old courses, or tributary courses that can be reoccupied during large floods. II. Anabranching. The most recent development of anabraching on the Luangwa River occurred in reach A between 1967 and 1983 immediately downstream of the Luangwa±Mupamadzi River confluence (reach A, location 1, Figure 3). In this reach, the Luangwa River flows over a very low gradient (`0Á0001) alluvial plain. It is joined by a number of tributaries that are highly sinuous due to low surface slopes. The anabranched reach was initiated at a minor channel which joined the main river immediately upstream of a meander apex (location 1). Here the Luangwa River breached its banks allowing water to spread over the floodplain scouring the minor channel and making it a permanent westerly anabranch. The eastern anabranch was similarly formed when the main channel was simultaneously shoaled as the water started flowing into the minor channel at the upstream end of a meander bend. This minor channel connected two consecutive bends. The scouring and reactivation of this minor channel in favour of the main channel caused the abandonment of the main channel and its subsequent filling with sand deposits (location 2, Figure 3). III. Meander development. (a) Outer bank development, rotation and translation. In many reaches of the Luangwa River following meander loop cutoff and realignment, stages of meander development can be observed on the aerial photographs. Hence at bends 18, 19 and 20 aerial photography captures the form of meander growth since channel straightening (Figure 8). Similarly in reach B increasing sinuosity between 1957 and 1967, then realignment and a new phase of meander growth are captured by the aerial photography (Figure 4). At bend 15 an acute bend had developed by 1956 with the first indications of concave bank development ( Figure 9A). Limited development of floodplain vegetation depicted on the aerial photograph also suggests a rapid migration rate and increase in curvature on this bend; lack of tree (b) Channel expansion, contraction and meander lobing. A typical river meander bend concomitantly erodes the outer bank and deposits on the inner bank. The process continues until, according to the model of Nanson (1975, 1984), the r/w ratio reaches a threshold (normally between 2Á0 and 3Á0) beyond which meanders tend to become constrained and cease to grow. For 1956For /57, 1967For and 1983, measurements from aerial photographs of r/w values for 22 meander bends were undertaken. All values were between 1Á4 and 6Á5. Between 1956 and1967 66 per cent of values exhibited an increase. Of those that experienced a decline in r/w all had a value of 2Á4 to 3Á9 at the earlier data. Between 1967 and1987 as many r/w values fell as rose. Lower values in 1987 occurred on meander bends with r/w value of between 2Á5 and 2Á99 in 1967 with the exception of one value (5Á18). This anomaly was for the same meander bend that produced the maximum value within the data set (6Á5 as recorded in 1957). In other words this meander did not exhibit a reversal to its radius of curvature at a value of between 2Á0 and 3Á0 in accordance with the findings of Nanson (1975, 1984), but became more tortuous and since 1967 has maintained this curvature. However, the bulk of the data suggests that on reaching an r/w value of somewhere between 2Á4 and 3Á9, but more often than not between 2Á5 and 3Á0, meander contraction and lobing occurs. At the stage when the meander ceases to grow it may either migrate down-valley, remain stable or reverse the meander development process by contraction. Previous studies suggest that double-heading of meander bends is more common than the reversal process which leads to the formation of concave bank benches (Nanson and Page, 1983;Hickin, 1979). The Luangwa River has an unusually large number of Earth Surf. Process. Landforms 25, 421±436 (2000) instances of bend contractions leading to the formation of concave bank benches. Four pronounced concave bank benches within the study reach are apparent (e.g. Figure 6). Bend contraction is reflected in marked differences in bankfull channel width between successive dates; contraction of channel widths by up to 50 per cent was observed, although overall between 1957 and 1988 channel widths increased by 20 per ...