Agreed. This is in the Nature section and the preferred translation is 'The power is scarce.' Duo has a done a great job putting the site together. But, they seem unwilling or unable to correct or revise weak examples, even when there is an out pouring of commentary that the example is problematic. Isn't that the point of soliciting commentary from users?
Knapp has a different meaning. All translations leo.org offers for narrow are meant in a physically describing way, like a narrow passage or to narrow it down on just a few options. Knapp on the other hand means something is rare or running out, 'Das Essen wird knapp' - We're running out of food or There's barely any food left.
In addition: If you wanted to say 'the stream is narrowing', that would be 'Der Fluss [or Flusslauf] verengt sich' or 'verjüngt sich' or maybe better 'wird schmäler'. Anyway, your 'stream' would be a big river in german which is, like electrical power, also called 'Strom' (homonym).