Maybe not the best big cat, Although I'm pretty GRRREAT, I hide so well that when you see me, It's probably too late

Add an s and I'll hold it together. Remove c and I let a dog drink. Replace p with d and I'll cover you. All together I'm thunderous. What am I?

I march in swarms, a hungry tide, My numbers vast, with nowhere to hide. I strip the land, then fade from sight, A plague of hunger, day and night.

I have a bed, but I never sleep. I have a mouth, but I never speak. I always run, but have no legs, Born in the hills from winter's dregs.

My elder scrapes the clouds. My younger kisses the plain. I stand between their stories. What am I?