I would first off think about the structure of your code and if you need that many standalone vars, maybe refactoring it to data.frame or so would be better? Anyway you could do something like this (only in a script though!):
Rhine_d1 <- sum(1,1)
Rhine_d2 <- sum(2,2)
Rhine_m1 <- sum(3,3) - Rhine_d2
env <- ls(.GlobalEnv)
var_names <- env[grepl("^Rhine_.*",env)]
bound <- data.frame(cbind(mget(var_names)))