To calculate the average gradient of the river, you can use the formula:
\[ \text{Gradient} = \frac{\text{Change in elevation}}{\text{Distance}} \]
In this case, the change in elevation is from 200 meters above sea level to 0 meters at sea level, which is a change of 200 meters. The distance the river travels is 400 kilometers.
First, we need to convert the distance from kilometers to meters:
\[ 400 \text{ km} = 400,000 \text{ meters} \]
Now, substituting the values into the formula:
\[ \text{Gradient} = \frac{200 \text{ meters}}{400,000 \text{ meters}} \]
Calculating this gives:
\[ \text{Gradient} = 0.0005 \text{ meters per meter} \]
To convert this to meters per kilometer, we multiply by 1000:
\[ \text{Gradient} = 0.0005 \text{ m/m} \times 1000 = 0.5 \text{ m/km} \]
Thus, the average gradient of the river is:
\[ \boxed{0.5 \text{ m/km}} \]