J Vos

I shred my leaves and have never had trouble with them blowing away.  I use them on perennial beds, in compost piles, and in leaf mold piles and all of them stay in place.

I have found that the best results come from beds oriented with their longest side to the south.  The sun moves from east to west and is lower in the summertime so your crops will get the most sun this way.  The greenhouse/high tunnel industry indicate that their structures should be oriented this way as well.

I like to use cedar fence pickets.  I stack them two tall and make the beds 4'x6'.  I buy the 6' cedar pickets and the ones I cut down to 4', I rip the 2' excess into pieces that can be used to secure the corners and add support on the sides of the bed.  Here is the basic plan I use and adjust to the size I want: https://ana-white.com/woodworking-projects/10-cedar-raised-garden-beds

I saw someone use garage door panels to make raised beds which I though was pretty brilliant.  They went to a local installer and asked for the old garage doors they removed and bolted them together into beds.  Seems pretty smart since they are steel and won't rot!

I'm going to try this method this year (fishing line fence):  

Swales might work for you but a simple solution such as a vetiver hedge might be just as effective.  It just depends on what your long term goals are.  It would definitely stop/slow the erosion.  Switchgrass might be another solution.  As far as the stream goes, check dams or small rock dams could be an answer.  Once again, a really simple solution that can have excellent results.  It is less labor intensive than a pond and can be tweaked until it fits your needs.