One good thing about winter is that you can see how all the drains work on your property and especially their shortcomings. If there are drainage problems then a proper plan is essential to fix them when the weather dries up enough to get machinery on to the job if needed.
Drains are not just a means of getting surplus water off your property. They can also deliver problems such as silt, weeds, trees and dead animals from neighbours' land up stream. Solving these issues will need a joint approach and cooperation between neighbours, and often the assistance and approvals of local authorities if major earthworks are needed.
Erosion is a special concern and it takes a great deal of skill to get the water levels right so main drains don't scour out and the banks don't fall in, often taking boundary fences with them. This can easily cause expensive legal problems as land can disappear.
With so much emphasis now on care of the environment and concern over fertiliser runoff, there's a great need for landowners to fence off drains. District and regional councils are actively promoting this, and the NZ Department of Conservation is also encouraging farmers to provide an environment to protect their local flora and fauna.
These organisations have great expertise worth consulting, as you need to be sure that whatever trees you plant will not cause problems ending up with more work such as cutting down willows that in in a few years time will stop water flow.
Drainage is not about getting rid of water fast, and there is much more interest now in recognising wet boggy areas as wetlands to slow up run off and trap nutrients. These become great features on the farm for bird and insect life and can be seen as an asset when in the old days they would have been top of the list for massive drains.
Special care is needed when spreading fertiliser near waterways, especially nitrogen and phosphate which is at most risk with bulk spreading. It's important to only use a ground spreading operator who is registered, and take time to show the operator any parts of the farm you are especially concerned about. Licensed ground spreaders are as anxious to do a good job as you are.
For protecting streams, there is a wide range of power fencing options available these days depending on what stock you need to keep out of the drained area. It's a good idea to leave the bottom wire dead as it will soon be covered by herbage and it will also allow safe access for hedgehogs. If you plant trees along the drain for soil stability and to shade the water surface, then they will need protection from cattle and horses until they get above browsing height.