Better have a look at that river sand before you plant too much in it! River sand can often contain high mineral content dangerous to many species of carnivorous plants. A good test is a drop or 2 of hydrochloric acid: if there is a fizz then the material may be unsuitable for CP cultivation. Sand needs to be quartz (silica) based. Sand containing lime (as in limestone) is quicky leached out by the acidity of the peat, and is concentrated at the surface by capillary and evaporative action, right where it can do the most harm.