# Characteristics of Channel Incision Induced by Human Activity in a Wandering Reach in 20 Years
## 1 Introduction  
River channel adjustments on the Earth are affected by various natural environment and human activities. In recent years, more and more studies have been carried out on the effects of human impacts on river channel evolution worldwide [1, 2, 3, 4, 5, 6]. Channel incision is one of the most important channel adjustments induced by anthropogenic interventions, and it is also considered as a quintessential feature of dis-equilibrated fluvial systems [7]. It may generate a series of environmental effects, including increasing flood hazards, undermining of structures, influencing water quality, and reducing habitat diversity [8, 9, 10, 11, 12]. Therefore, a better understanding of channel incision in response to human interventions can help to mitigate detrimental effects and to predict channel evolution in the future. Many anthropogenic interventions can trigger channel incision in alluvial rivers, such as dam construction, gravel and sand mining, and levee and embankment construction [13, 14, 15]. Previous studies indicate that dams' and reservoirs' operation can lead to a dramatic depletion of sediment load, which may result in severe channel incision downstream [16, 17, 18, 19, 20]. Gravel and sand mining in rivers may change the position of knockpoints upstream, and the rate of gravel and sand mining is greater than that of sediment replenishment, which leads to significant channel incision [10, 21]. The effect of lateral structures on channel incision is different from that of longitudinal structures, and the levee has more effect on channel incision at the early stages [1]. Many previous studies have been carried out to investigate the extent, influencing factors, and effects of channel incision by field observations. For example, [PERSON] et al. [10] investigated the process of channel incision and thechanges in river hydraulic quantitatively in the lower Pearl River, and discussed the effects of sediment reduction induced by natural environment and sediment extraction induced by human activities. [PERSON] et al. [12] considered that the lowest annual water level should be an indicator of channel incision, and it can be affected by river size and channel lateral mobility. [PERSON] et al. [20] compared bathymetric data of the Lowermost Yangtze River in 1998-2013, and it is found that the observed depths of channel incision were up to 10 m at many locations owing to the Three Gorges Dam operation. [PERSON] et al. [11] evaluated the associations between channel incision and water quality, and it indicates that the total phosphorus, total Kjeldahl N, and chlorophyll a concentrations were significantly higher in the incised stream than in the non-incised stream. Some researchers also developed models to predict the potential channel incision in rivers. For example, [PERSON] et al. [22] developed a model using several topographic indices to predict the distribution of channel incision in gullies. [PERSON] et al. [23] identified the most influential factors of channel erosion and proposed a model to delineate the high and low susceptible areas in relation to gully incision. [PERSON] et al. [24] developed an algorithm that can not only identify existing incised features, but also predict areas at risk of future channel incision. These models can predict the potential channel incision spatially, but they cannot reflect the process of channel incision varying with time. The Xiaolangdi (XLD) dam was built at the outlet of the last canyon in the Middle Yellow River in 1999, and it was constructed for controlling floods, reducing sedimentation, generating hydropower, and irrigating farmland [25; 26]. A water-sediment regulation scheme began to be implemented every year by the Yellow River Conservancy Commission of China (YRCC) in 2002 [27].