Outside of needing dams for water supply during dry months, and helpful for generating energy, but what are their impacts? Well from interestingengineering.com, I stumbled upon this article that address what are the good and bad about Dams:

"Dams cause earthquakes but they also produce much-needed energy.

Did you know that China’s Three Gorges Dam is so massive that it slows the Earth’s rotation? When the dam is at full capacity, its reservoir holds 42 billion tons of water which is actually enough to slow the planet’s rotation, increasing the length of a day by 0.06 microseconds.

This small fact illustrates just one of the many ways dams reshape our world. Dams have both good and bad consequences, and in this video, we explore them all.

Let's start with the bad. Between 2000 and 2009, more than 200 dam failures occurred across the world, triggered by everything from earthquakes to human error. There are also more than 100 cases of earthquakes that have been caused by dams.

However, there aren't just dam failures around the globe, there are many dam successes. Reservoirs created by dams help control flooding. They fill valleys in a manner that allows them to be used for boating, fishing, and other human recreation. Dams also provide much-needed electricity.

So are dams good or bad? This question is hard to answer but in certain areas dams are actually quite necessary."

Yet first lets define what a Dam is and is not:

"What is a dam?
A dam, as defined by the British Dam Society is:

"A barrier or structure across a stream, river, or waterway to confine and then control the flow of water. Dams vary in size from small earth embankments, often for farm use, to high massive concrete structures generally used for water supply, hydropower, and irrigation."

What are some record-breaking dams?
Dams are amazing structures in and of themselves. But some among them stand out as truly outstanding works of engineering.

For example, the highest dam in the world, as recognized by Guinness World Records, is the Jinping-1 Dam in Sichuan, China. She is a whopping 1,000 feet (305 mt) tall — taller than the Eiffel Tower in Paris.

The smallest dam in the world, according to some sources, is the Inks Dam on the Colorado River. She stands at around 97 feet (29.4 meters) tall and is about 1,550 feet (472 mt) long.

The deepest dam in the world is widely considered to be the Parker Dam on the Lower Colorado Basin. Built with 380,000 cubic yards (290,531 cubic meters) of concrete, the dam has a height of 320 feet (98m), 73% of which is below the original river bed. Only around 85 feet (26m) of the dam's structure is visible. "

One noted reasons for Dams are to produce energy here is a bit on that as well:

"Did you know that 16% of the world’s electricity is produced through hydroelectric power plants, most of which are located inside, on, or near dams? Dams are indeed very useful marvels of engineering but we don't often think of the fact that they actually serve to produce energy.

It might seem confusing since at first glance these large constructions seem to only have one purpose: to keep water from flooding areas downstream. However, most dams do indeed support the hydroelectric power plants that produce our precious energy by taking advantage of one simple thing: gravity.

They do this with the help of wind turbine generators which harness the kinetic energy of the water as it falls from high up behind the dams. These generators convert kinetic energy into mechanical energy which is then further converted into electrical energy with incredible efficiency.

In fact, hydropower plants can convert energy from the water held by a dam into electricity with up to 95% efficiency. Wow!"

Other advantages of Dams are:

Flood control:
Dams are an effective way to control and regulate the flood waters so that both life and property are not damaged as a result. During periods of excessive rain such as cloudburst or heavy rain, the dam can help store the excess water which can be released back into the river under controlled conditions. It can also help divert the water to other areas and thereby help provide the same with some essential water supply.

Recreational:
Even though dams are not constructed for the purpose of recreation, the fact remains that it can be marketed to others as one and the revenue earned can go towards the maintenance of the dam. In fact, most dams do charge an entry fee for allowing the public to view the dam and all the features. Some have even integrated several recreational facilities that cater to the public. So if you are planning to visit a local dam you may want to check online and see if the place in question offers boating, water skiing facilities or something along those lines.

Irrigation:
For countries that depend on seasonal rainfall such as monsoon for their water, need to have dams in place that can help store the essential water. Dams can help store the water safely so that it can later be released and used by the local population to meet their irrigation requirements.

Electricity generation:
Dams help to produce cheap electricity; granted setting it up may seem like an expensive exercise but the fact remains that the power generated is clean, free and the dam should help pay for itself over the years. The US currently produces over 105000 MW of renewable energy in the form of hydropower which meets 8 to 10 per cent of the nation’s requirements.

There are a number of down sides of Dams and depending on your political leanings they can be vastly different, but to remain neutral on the subject and let you decide here are some disadvantages across many different web sites to give you a good picture of the down sides of Dams:

Environmental damage:
Often building a dam in a particular area requires flooding vast swathes of land, submerging the entire local ecosystem under water permanently. This has an impact on the local environment and especially on several species that are dependent on natural habitats for their very survival. Furthermore, this also requires all local inhabitants to move to higher areas and more often than not, the local population are not compensated adequately to make up for the loss of their ancestral lands or even a newly built home.

Fish:
One of the negative aspects of any dam is that it often prevents fishes and other organisms from migrating to their natural breeding grounds. And while several studies have indicated that some of the affected species have found new grounds, the fact remains that the population levels have started plummeting of late.

Risk of floods:
One of the positive aspects of dams is that it prevents floods and ironically, this is the same reason to be wary of dams. Dams that are old are subject to extreme pressures for long periods of time and as a result, they may have developed structural damage. To date, not many countries bother to check out their dams in detail to assess if the dam in question has structural damage or not. As a result, even a simple earthquake can cause the dam to collapse, flooding the entire valley, and causing fatalities. Incidents such as these have happened in recent past, such as the Banquet and Shimanten Dam collapse in China which caused the death of over 171000 people.

Biodiversity Impacts:
Flooding landscapes to create reservoirs can have consequences for biodiversity as well.

Thus the questions is do you consider Dams good, bad, or a mix of both? Without Dams we could be facing a number of larger problems, but they do come as a cost. Thus what is the Risk to Benefit of Dams? Personally I think they are a good thing where the benefit is greater than the impacts in relation to mankind. And yet there is room for improvements as technology improves to lessen the impacts to the Environment and other life forms.

References:

https://interestingengineering.com/video/how-dams-are-reshaping-the-world-for-the-better-and-worse
https://interestingengineering.com/video/the-almost-mystical-way-dams-produce-energy
http://importanceofstuff.com/dams
https://greengarageblog.org/17-biggest-advantages-and-disadvantages-of-dams
https://www.positivenegativeeffects.com/dams