Throughout August-September, there tends to be a lot of rooted aquatic plant (macrophyte) growth. These plants grow rapidly as they photosynthesize due to warmer water temperatures. Dense mats of aquatic plants displace water, making it appear like the depth is greater.

Why does it seem like streamflow can be so high in the late summer/early fall?

Think of the river as a bathtub. With a bathtub, the water level seems to increase when you step inside, as your weight displaces the water. It is the same with the river. The plants displace river water, making the river appear to be more full than it really is.

How long does macrophyte growth continue?

On average, macrophytes will continue to grow through early-to mid-September, but this is dependent on the weather. Once growth has peaked in the fall, macrophytes begin to decay and deteriorate, causing bits and pieces of old plant growth to float down the river. This is a normal process, and some of the species on the Henry’s Fork actually propagate in this way.

What are the implications of macrophyte growth?

• Macrophyte growth is an indicator of increased aquatic ecosystem productivity.
• Fish utilize macrophytes for shade and as a refuge from predators.
• The photosynthesis of plants helps to create more dissolved oxygen in the river, which is good for fish.
• Macrophytes act like a big filter, trapping fine sediment during the growing season. This sediment cannot be scoured or exported while the plants are still there (which is typically July- February).

Why does the USGS stream gage sometimes read slightly higher/lower than the “actual” flow of the river?

Aquatic vegetation at the Island Park stream gage reaches its peak for the season during the later summer/early fall. During this time, the vegetation begins to die and move out of the channel. That means that for the remainder of the fall, the gage will read slightly lower than the “actual” flow in between rating adjustments. Flow adjustments take place approximately every 6 weeks.

What can HFF do to remove sediment that gets trapped by macrophytes?

Once the macrophytes are gone, there is a limited window of time when the river can be scoured of fine sediment (March, April, May, June). The solution is to run a spring freshet (rush of fresh water at ~2,000 cfs) to help export this sediment.

For more information regarding the April 2023 spring freshet, check out HFF’s water quality website: https://henrysforkdata.shinyapps.io/Freshet2023/