Final answer:
The higher population densities in areas around river deltas and upland lake and river valleys can be attributed to fertile soils, access to water resources, and trade opportunities.
Step-by-step explanation:
The higher population densities in areas around river deltas and upland lake and river valleys can be attributed to several factors. First, these regions often have fertile soils, which promote agricultural productivity and can support larger populations. Second, the presence of rivers and access to water resources makes these areas suitable for farming and other economic activities. Finally, the availability of trade routes and access to international trade can further contribute to the population growth and development of these regions.