When designing and building a stream, firstly plan for it to be lower than the surrounding ground.

Then consider, what impact will the flow of pumped water have? Answer: it will raise the water-level.
The amount of water generated by this pump is too much for the banks of the stream to contain.
Or put another way, the stream is too flat, too narrow and too shallow for the volume of water flowing from the pump.
The consequence is multiple-overflows of the banks, which is what happened here.
 (This phenomenon applies equally to waterfall lips between ponds,
and it occurs more often than not with novice construction.)




Other Design factors you might like to consider:

 - Where should I site the pump for easy access:  to make flow adjustments, to maintain it and later to
    replace it after years of use.

  - Where to site the feature for sun orientation and maximum viewing pleasure.

  - Do I know enough techniques to conceal my pump, and pond-edges from view.

  - Can I make it entirely kid-safe? (yes you can!)

  - How do I design the pond stream and pump location for maximum water clarity and minimum
    maintenance.

  - How do I select the correct size/volume pump for the water effects I want?

  - Can I introduce different moods to the water feature? (fast, slow, swirl, mysterious origin)

  - Location of electrical outlet

  - Location of overflow point and drainage to where.


If you are unsure about too many of these factors, I recommend you invest in a Design Report for general planning, or a "Day's Assistance" from me on site, to help put all these factors in place.