Final answer:
The main topic is the effect of draining uplands for farming on soil stability, erosion, and changes to river channels and floodplains. Draining can increase erosion, and soil washed away can alter natural floodplain landscapes. It also emphasizes the importance of soil management in maintaining land productivity.
Step-by-step explanation:
The main topic of the excerpt above is the impact of draining upland areas to increase their suitability for farming in terms of soil stability, erosion, and changes to river channels and floodplains. Draining these areas can decrease soil stability, leading to increased soil erosion when it rains. The soil that is washed away adds to the river's load, resulting in changes to the natural state of the downstream floodplain landscape due to enhanced deposition.
Intensive cultivation and inadequate soil management can lead to reduced soil organic matter content, which in turn diminishes the soil's capacity to retain water. This can provoke floods, further erosion, pollution, and more organic matter loss. Differences in topography, such as steep slopes compared to flat areas, greatly affect rates of soil erosion, runoff, and water infiltration. The direction of slope faces can also determine soil moisture and erosion rates, with south-facing slopes in the northern hemisphere being warmer and drier, thus more prone to erosion.
In some regions, farming practices have led to a significant decrease in soil productivity, either through erosion or salinization. Well-drained soils, which tend to be on hills or slopes, generally have different characteristics from poorly drained soils, which are often found in lowlands. The practices around riverine systems, such as those in Mesopotamia, show historically how farming depended on the nutrient deposits from flooding rivers, yet modern practices without sustainable soil management can quickly deplete soil nutrients.