River's goal is to become ONE with the ocean

If you stand on a river bank, are you watching the same river? 

Yes. 

Is it the same water?

No. Water that you watched a moment before at some particular location has moved on and new water has come in it's place. 

Can you find the difference between them? 

Not easily unless say muddied flood water comes along and replaces the clean water before. Even in that case, change is expected to be pretty gradual and slow. Hardly perceptible.

So is human evolution. Consciousness in us is like the water. We are the physical river. We, the body, remain same but the consciousness in us keeps flowing. Newer and newer consciousness replaces what was before. That's how the personality changes. As years pass, river also erodes along the banks. Our bodies also erode as we age. What a beautiful analogy! (not sure where I read this but stuck in my mind).

We see the same person. May be the body undergoes some change but the essence in it is continuously changing very much like the new water replacing the old water. That's the reason why we sometimes find people have changed a lot over the years. Sometimes for good and sometimes for bad.  Here change refers to changes in personality or outlook. A particular body of water accumulates a lot of things in it as it flows through different terrains. Those accumulations become part of its essence. If you take a sample of water in some location, it may taste one way due to certain elements and taste totally different if the same water is taken a few miles up or down the river. Similarly the consciousness in us undergoes several changes and gets colored by different experiences over the years.

But, there is a catch. Even if we think that someone has changed for worse, in strict 'karmic' sense they have evolved. Remember, river flows only towards the ocean and not away from the ocean. Similarly, everyone evolves towards the perfection and not away from it. A particular change in a person may seem like a change for worse to our untrained eyes but it is like saying that certain part of river is not good just because it's dirty. As long as it's closer to the ocean, it does not matter.

Whether you try or not, you are moving towards the ocean. Becoming one with ocean is the goal of every drop of water in the river. A drop of water at the source of the river has several hundred miles to travel, accumulate a lot elements (i.e. experiences) before it becomes one with the ocean. A drop of water which is at the mouth of the river has only a few miles before it becomes one with the ocean. Regardless, every drop of water that is generated at the source is destined to become one with ocean. Similarly some people are relatively closer to realization and some are relatively  away. But everyone is surely moving towards it.

Even if some water in the river evaporates or taken away from river in some fashion, it transforms to some other energy form and finally dissolves into basic elements and to energy.

Sometimes we see whirlpools in rivers. When water gets into them, it stays swirling there for a long time before flowing on. Similar humans also get stuck in karmic whirlpools from time to time. That's part of the journey towards the ocean of oneness. Have patience. It helps to deal while you are in the whirl pool.

Strictly speaking, this analogy is not fully correct from Vedanta standpoint because consciousness in us pure and same and does not change. 

But, simple analogies are given to make a point. Advaita vendata is based on a very simple principle. Use lower truth to get closer to the higher truth. This analogy of river is probably on the lower end. 

Cheers!