2.1. Study Area
The TGDR region was characterized into three zones, subject to their land-use: rural areas, rural–urban transitional areas, and urban areas. The sample transects were chosen from the reaches of rural, rural–urban transitional, and urban belts within the riparian zone of the TGDR territory (Figure 1). The investigated sites spanned a gross area of 45,100 km2 involving 15 counties across two provinces (Chongqing and Hubei) of China and stretched from Jiangjin county to Zigui county (31°2′34.0″N, 109°33′41.0″E). These counties form part of the humid subtropical climate zone, with monsoon weather. The mean annual air temperature of the sample locations was 17.5 ± 1.2 ℃ (mean ± standard deviation), and annual rainfall was 1,160.9 ± 118.7 mm (Figure 2). Morphologically, the maximum and minimum altitudes of these areas were 1,926.7 ± 683.8 m above sea level (a.s.l.) and 115.2 ± 45.0 m a.s.l., respectively. The length of the sampled streams was 68.1 ± 29.3 km. It is worth mentioning that the water level varies throughout the year, with a typical yearly flow of 10498.33 ± 851.87 (m3/s) (Figure 3). This area was under broad-leaved and mixed forests, with croplands and gardens also present in several sites. Soil maturity and stability were lower owing to rock weathering in the newly formed riparian areas, and soil made of calcareous purple sand shale was found in the area (Li et al., 2021; Zheng et al., 2021a). Consequently, erosion activities were more intense at rural sites, whereas more concrete structures were found in urban and rural–urban transitional locations.
[Figures 1–3 to be inserted about here]
The pattern of RHIs was irregular in the transects of rural, rural–urban transitional, and urban belts, as well as their subsets. Similarly, pressure indicator sequences were inconsistent, depending on the geographical sites. A unique vertical division was observed among RHIs and pressure indicators across the TGDR geographical locations. Generally, the lower banks were quite exposed, and thin grass strips were present in small patches. These areas were relatively sandy and steep, and fluctuating waves exacerbated the erosion process from the navigational events across the whole reservoir. Erosion was evident in the rural and rural–urban transitional areas, although grasses entrenched in the middle bank areas helped naturally stabilize the banks. Organic litter was not detected owing to the absence of understory vegetation on the ground of the middle banks. Scattered understory and canopy cover on the upper banks kept these areas relatively stable. During field visits, the research team noted various sites used for sustaining the ecological integrity of terrestrial and aquatic ecosystems under different ecological restoration projects throughout the TGDR riparian zones. Humans have played a significant role in changing the environment and land use of the TGDR through deindustrialization, suburbanization, cultivation, fishing, and other ecologically unfriendly activities. Therefore, different pressure indicators were recognized in most of the sites within the study areas. All these factors were collectively responsible for deteriorating the riparian health conditions of the TGDR.