To understand this question, it's important to take a look at the definition of velocity. Velocity is a measurement of the rate of change of speed and direction of an object (in this case, a stream). Velocity is a vector quantity, but this question asks you to consider the absolute value (scalar magnitude) of the velocity vector, which is speed.
The formula for velocity is:
v = velocity (sometimes denoted with r for rate)
d = distance
t = time
However, a stream isn't moving in one straight line at a single rate of movement. On top of that, additional influences such as gravity play a huge role in calculating stream velocity. To answer this question, you will need to consider Bernoulli's Equation:
1/2ρv^2 + ρgh + p = constant
ρ(rho) = density
v = velocity (of the stream)
g = acceleration due to gravity
h = elevation
p = pressure
From this, you can see that a decrease in velocity will signify an increase in density and/or pressure. Of course, you can also imagine that a decrease in velocity is an effect of a decrease in elevation (gravitational acceleration from potential > kinetic energy).
Please let me know if you have follow up questions and/or if any of this explanation is unclear. I'm happy to clarify and/or expand on this response.