In physical geography and hydrology, which of the following is a major natural cause of floods in river systems?

Introduction / Context:
Floods are natural disasters that occur when river water levels rise and overflow banks, affecting surrounding land and settlements. Understanding the natural causes of flooding is important for students of geography and environmental science as well as for general awareness. This question asks which factor among the options listed is a major natural cause of floods in river systems, focusing on hydrological processes rather than human actions.

Given Data / Assumptions:

• The topic concerns natural causes of flooding in rivers.
• The options include light rain, reduced stream discharge, rapid spring snowmelt, and increased channel capacity.
• We assume no artificial factors like dam failure or urban drainage are being considered.
• We also assume the question is framed for typical mid latitude or mountain fed river systems.

Concept / Approach:
Floods often occur when the volume of water entering a river exceeds the capacity of its channel. Rapid melting of snow in spring can release a large amount of water in a short time, especially in rivers fed by snow covered mountains. This sudden influx leads to swollen rivers and flooding downstream. Therefore, the correct approach is to identify which option introduces an increase, not a decrease, in water volume that could realistically cause a flood.

Step-by-Step Solution:
Step 1: Examine each option in terms of its effect on river water volume.Step 2: Light rain over a large area usually adds water slowly and may not be enough to cause severe flooding by itself.Step 3: A decrease in stream discharge reduces the volume of water, which would reduce flood risk rather than increase it.Step 4: Rapid spring snowmelt suddenly adds large amounts of water to rivers, especially in mountainous regions, which is known to cause floods.Step 5: Increased capacity of stream channels means the river can hold more water, which reduces flood likelihood, not increase it. Therefore, rapid spring snowmelt is the correct answer.

Verification / Alternative check:
Verification can be done by recalling well known examples such as snowmelt driven floods in Himalayan rivers or rivers in temperate regions after mild winters followed by sudden warming. Hydrology textbooks often list snowmelt as a primary factor in seasonal flooding. The other options either reduce the risk of flood or are less intense triggers compared to rapid melting of accumulated snow.

Why Other Options Are Wrong:
Option A (light rain over a large area) is typically insufficient to cause major flooding because the rainfall rate is low and spread out over time. Option B (a decrease in stream discharge) refers to lower flow in the river, which cannot logically produce floods. Option D (increased capacity of stream channels) indicates that the river can carry more water before overflowing, so it actually helps prevent floods rather than cause them.

Common Pitfalls:
Some candidates may misinterpret “light rain over a large area” as a strong cause of flooding, forgetting that intensity and duration of rainfall matter more than the area alone. Others may quickly choose any option mentioning “increase” without considering whether it increases water volume or channel capacity. To avoid such errors, students should remember that floods are mainly about too much water entering the channel too quickly, which is exactly what happens during rapid spring snowmelt.

Final Answer:
The correct answer is rapid spring snowmelt, a major natural cause of floods in many river systems.