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.