Island dynamics in a braided river from analysis of historical maps and air photographs

Abstract

An analysis of island and active corridor dynamics is presented for a 16 km island-braided reach of the gravel-bed Tagliamento River (Italy) based upon information extracted, geocorrected and registered to a common base from three map The active corridor width showed a general decline over the study period but with some recent widening. Adjustments in active corridor width were achieved through processes of floodplain avulsion, island attachment and progressive encroachment of the edge of the active corridor across gravel areas. These adjustments were accompanied by the preferential creation of dissection (floodplain avulsion) islands during periods of widening and the construction of mid islands within the corridor during periods of narrowing. Changes in island extent were achieved by rapid island turnover, which reached a maximum rate of over 50% per annum when corridor narrowing was most rapid between 1970 and 1991. Very few island surfaces were found to persist for more than 24 years.Despite this enormous dynamism and apparent cyclic behaviour, between 1944/6 and 2005 the ratio of island area to active corridor area remained relatively constant at around 0.08 and supported a consistently high bankfull shoreline to downstream length ratio of around 6 km Á km À1 . These intrinsic properties of the dynamics of the study reach and other island-braided channels need to be recognized and maintained by river managers because they represent a characteristic habitat dynamism that is crucial to the maintenance of ecological integrity.These areas are calculated as the area of changed cover between surveys divided by the number of years between surveys and then expressed as a percentage of the area that persists as island between the two survey dates.

Physical modelling of the combined effect of vegetation and wood on river morphology

“…In systems where deposited wood can sprout to form new trees, the cycle is tighter and quicker, extending over multiple decades rather than centuries, and thus is able to recover more quickly (e.g. Zanoni et al, 2008). Nevertheless, the crucial contributions of wood and trees to river ecosystems need to be recognised, and their joint conservation needs to be incorporated where possible into river management.…”

Variabilidad espacial de los cambios de cauce en una rambla mediterránea en las seis últimas décadas (1946-2006)

“…10), causing an enlargement of the gravel cannel. These destructive processes related to the occurrence of high magnitude floods or to a high frequency of significant flow events have been documented in other Mediterranean rivers (Zanoni et al, 2008;Piégay et al, 2009;Rinaldi et al, 2009;Surian et al, 2009Surian et al, ). -1978Surian et al, -1991.…”

“…The margins of each form were manually digitized; and an attribute table was created for the resulting polygons. The channel forms were classified following a modification of the conceptual model of Gurnell et al (2001) and Zanoni et al (2008). We distinguished between: (i) gravel channels, unvegetated branches of the river bed, frequently affected by flow; (ii) incipient islands, gravel or cobble patches covered by less than the 20% scattered bushes and sparse grass cover; (iii) established islands, covered by more than the 20% scattered bushes or trees and completely covered by a dense grass layer or occupied by crops (olive trees or vines); and (iv) dissected floodplain islands, the remaining parts of the floodplain incorporated to the active corridor by chute cutoff processes.…”

“…Although underrepresented, the chute channels have been separated from the gravel channel category because they indicate processes of floodplain dissection. The outer limit of the active corridor was defined by the boundary between areas of gravel and any remaining, extended vegetated surface that had not been classified as island at any period (Zanoni et al, 2008 …”

Channel Planform Dynamics Monitoring and Channel Stability Assessment in Two Sediment-Rich Rivers in Taiwan

“…Techniques of remote sensing have been widely applied to river change studies. Based on aerial photograph analysis, researchers can join traditional maps to analyze long-term island changes and channel expansion/narrowing [7], or combine field surveys to evaluate the impact of changing sediment flux on braided river evolution [8] and also to observe sediment pulses after bend cutoffs in wandering rivers [9]. Moreover, remote sensing from satellites can capture recurrent images and have multi-spectral sensors, becoming a powerful tool to monitor changes on the earth's surface over a large area.…”