Accelerated overbank accumulation after nineteenth century river regulation works: A case study on the Maros River, Hungary Tímea Kiss a, ⁎, Viktor Gy. Oroszi a , György Sipos a , Károly Fiala b , Balázs Benyhe a a Department of Physical Geography and Geoinformatics, University of Szeged, Szeged, 6722, Egyetem u. 2-6, Hungary b Directorate for Environmental Protection and Water Management of Lower Tisza District, Szeged, 6722, Stefánia 4, Hungary abstract article info Article history: Received 24 January 2011 Received in revised form 2 August 2011 Accepted 12 August 2011 Available online 22 August 2011 Keywords: Overbank sedimentation Regulated floodplain Accelerated accumulation DTM Invasive plant species In the nineteenth century, the meandering rivers of the Carpathian Basin were extensively regulated, and their large (up to 100 km wide) floodplains were confined by artificial levees. On the narrow (0.5–4 km) artificial flood- plains confined by these levees, overbank sedimentation has become the dominant geomorphological process, resulting in rising floodplain levels during the last century. The Maros River is an extreme example of this process, as it carries a significant amount of suspended sediment (4.6 × 10 6 m 3 /y) and because its lowland reach was dras- tically shortened by cutoffs, resulting in bed scour and an increased sediment load. Simultaneously, a flood em- bankment system was constructed, reducing the floodplain width by 70–80%. The resulting accelerated overbank sediment accumulation increases the flood hazard along the river. The aim of this study is to determine the spatial and temporal pattern of overbank sediment accumulation on the artificial floodplain of the lower Maros River over the last 150 years and to identify its influencing factors. The lateral slope of the protected (natural) floodplain was projected towards the present-day channel, allowing the volume (m 3 ) and rate (cm/y) of overbank sedimentation to be calculated using digital terrain modelling (DTM). Changes in the rate of sedimentation were determined by pollen analysis. The studied reach was divided into five sections based on the rate of aggradation: (i) the distal surface of the alluvial fan, where no sedimentation was measured, only incision; (ii) the original front of the alluvial fan (1.0 ± 0.4 cm/y); (iii) the recently formed secondary alluvial fan (1.4 ± 0.3 cm/y); (iv) the low-lying floodplain (0.4 ± 0.2 cm/y); and (v) the outlet unit (2.0±0.5 cm/y). The aggradation rate was slower on elevated forms, such as the pre-regulation natural levee (0.2 cm/y), and greater on the low-lying floodplain (0.6 cm/y). The greatest accumulation rate (1.3– 2.4 cm/y) was measured in cutoffs. The sedimentation rate was high during the regulation period (1.9–2.4 cm/y) but later decreased to 0.5–0.9 cm/y. The rate of aggradation was determined by the width, height and slope condi- tions of the floodplain, the impoundment of the main river and the land use in the area. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Floodplains are widely studied all over the world because of their importance in sustainable river management, especially in densely inhabited plain regions, where flooding is a major hazard. Building of embankments and levees has always been a first-order approach to decreasing flood risk. However, these constructions can provide a false sense of security, as hazard and risk may increase over time as the floodplain experiences accelerated fluvial processes. One of these is overbank sedimentation, which can decrease the drainage ca- pacity of the artificial floodplain, thus contributing greatly to the in- creased frequency of flood stages in recent years. Overbank sedimentation is influenced by different regional and local factors. Sediment influx is a key factor, but the sediment discharge is dif- ferent in each flood, so the rate of sedimentation is not proportional to the magnitude or frequency of flood events (Magilligan et al., 1998; Sándor and Kiss, 2006). Upstream inundation conditions (or levee fail- ures) also influence the downstream sedimentary processes (Kiss et al., 2002). The most important local factors controlling the transport and depositional environment of the floods are stream gradient, stream power, and the duration of inundation (Middelkoop and Asselman, 1998). The vegetation of the floodplain influences flow velocity by in- creasing surface roughness (Kiss and Sándor, 2009; Schenk and Hupp, 2010), while channelisation alters the connectivity of the floodplain (Kroes and Hupp, 2010). The relationship between overbank sedimen- tation and floodplain elevation and distance from the channel are un- certain (Hupp and Morris, 1990; Middelkoop and Asselman, 1998; Oroszi, 2008; Schenk and Hupp, 2010). Moreover, in narrow floodplains, the deposited sediment could be remobilised, influencing long-term sedimentation rates (Phillips et al., 2007). The spatial and temporal combination of these natural factors results in a rather Geomorphology 135 (2011) 191–202 ⁎ Corresponding author. Tel.: + 36 62 544545; fax: + 36 62 4544158. E-mail addresses: kisstimi@geo.u-szeged.hu (T. Kiss), viktor.oroszi@gmail.com (V.G. Oroszi), gyuri@earth.geo.u-szeged.hu (G. Sipos), FialaK@atikovizig.hu (K. Fiala), blarance@gmail.com (B. Benyhe). 0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.08.017 Contents lists available at SciVerse ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph