The depth and flow of water has direct impact on the river ecology. In simple word we can say like this, when the depth increase means decreasing light penetration capacity which essential for photosynthesis. The higher water velocity reduce the self cleansing mechanisms of the river. But it can create larger re-aeration rate when the water flow at higher speed.

Under this consideration, The rate of flow directly influences the quantity of oxygen dissolved in the water. The quantity of sediment and debris in a river can carry also depends on its flow rate and thus the higher water flow rate increases river bank and stream channel scouring and erosion. It also keeps particulate materials suspended in the water.

correspondingly, It has been due to that most things living in the deep ocean are mainly water and thus water is incompressible. Without gas-filled spaces like lungs or swim bladders, organisms in the great deep are less influenced by pressure more than than we imagine. The Deep-sea pressures do influence the rates of reaction chemical reaction somewhat, speeding up reactions.

Certainly depth and flow are among factors as suggested by Dr. Tesseme. But complicating your study of two parameters are many others that contribute to aquatic habitat, productivity, etc. A few that come to mind are substrate, woody debris, stream temperature and water quality. And there are many types of aquatic organisms that are adapted to a range of conditions from those considered or classified as intolerant of pollution or poor water quality to those that tolerate poor water quality conditions. I don’t suppose this is much help. Many aquatic organisms are mobile enough to be able to move their location to one of safety if flow velocity or depth becomes a limiting factor. When temperatures in the river become too high, fish may seek out the cooler waters entering from tributaries or deep pools with groundwater, hyporheic input . During high flows, aquatic organisms may seek safety in the substrate, or release and float to a more acceptable velocity and habitat. Aquatic organisms sometimes release and drift in cover of night especially if habitat conditions become too challenging. Within a stream reach, there may be riffles, runs, pools and glides that help to provide a range of habitat and conditions for varying needs. Water quality can play a big role in aquatic productivity, such as high alkalinity of karst geology and spring fed waters. In your data collection, you might consider the approach of Montgomery and Buffington, as they record factors as channel width, depth, gradient, substrate, any fish observed, etc. to survey habitat types. A fact that surprised me was the high fish habitat classified by fish biologists was in Dry Creek, coastal Oregon, was in a section of river with coarse gravels and cobbles that appeared ephemeral to intermittent, but maintained perennial intergravel flow important to salmon and steelhead eggs and rearing. And in North Carolina, the USFS fish biologist showed us various darters (some considered endangered) found in relatively high gradient headwaters observed by snorkeling. Some species are adopted to what might be considered extreme as cascade velocities, but they most likely find some niche, protection of substrate, boulder or rock ledge undertoe or eddy margin that is within their range of tolerance.

correction, fish biologist Jim Hurd from USFS Tennessee. He was a major contributor to the Chattanooga Fish Aquarium, and helped with their darter exhibit, but showed us also his inventory methods in the field using snorkeling, underwater movies, etc.

Water depth and velocity are some of the important factors for abundance, richness and distribution of aquatic organisms. Water depth is very important for colonization of aquatic insects as the temperature, vegetation, velocity, turbidity vary between the deeper and shallower water. Water velocities are not the same in different depths. Highest velocity occurs below water surface, middle depth have lower velocity and the lowest velocity occurs in benthos section. The velocity of moving water influences substrate particle size. Substrates may range from large boulders to fine sediments resulting in a wide range of microhabitats. The transport of inorganic and organic materials by the water current may be either detrimental or beneficial. Increased velocity causes erosion of silt and sediment and sometimes wash away the aquatic organism's colony. The range of current velocity also increases habitat diversity and various taxa are adapted to different velocities. However, fast moving streams have higher level of Dissolved oxygen hence better aerated than slow moving streams.