How to give names/labels to topics in LDA
I suppose it is late to answer, but it may turn out to be helpful for others.

LDA presents all topics as a mixture of keywords with their weights
For example, in gensim implementation it looks like this:

topic #0: 0.009*river + 0.008*lake + 0.006*island + 0.005*mountain + 0.004*area + 0.004*park + 0.004*antarctic + 0.004*south + 0.004*mountains + 0.004*dam

So you may name a topic using the keywords with the largest weights (like River+Lake).
Or you may use something like WordNet to find the most common hypernym for them.
For example, in python you can do it like this.

from nltk.corpus import wordnet as wn
wn.synset('river.n.01').lowest_common_hypernyms(wn.synset('lake.n.01'))