Lower Charles River Flood Plain and Alluvial Soils

With this update, I will examine the lower Charles River Watershed. I briefly touched on the upper and lower Charles River Watersheds in a previous post. The division between upper and lower is drawn at the Watertown dam. Below is a map by the EPA showing the division and boundaries.

Image Source: EPA

The lower Charles River Watershed is dominated by development and heavy human use as the potion of the watershed that contains parts of Boston proper. As you can see, almost the vast majority of the lower basin is actively used by people for residential, commercial, transportation, or recreational uses. There are small patches of marshes, reservoirs, smaller streams, and the river itself but the land is largely developed and impermeable.

Image Source: Image Source: Zarriello, P. J., & Barlow, L. K., 2002

The streams, river, and runoff do deposit sediment for the soil in the lower watershed, but much of the area especially towards the city is artificial fill and urban land (Weiskel, P. et al 2005). The artificial shaping of the land near the city is a longstanding tradition and began in the 1600s when the first European settlers began expanding the city by leveling some of it's famous hills to fill in the land between what was a peninsula and the mainland (Mitchell, J. H. 2008).

Image Source: Zarriello, P. J., & Barlow, L. K., 2002

Below, you can see a map of what the shoreline would look like before the deliberate filling of the bay. Many of the places I visit everyday would simply be water or marsh.

Image Source: Weiskel, P. et al 2005

It is hard to imagine these areas without human development, and like it or not the land has been changed. To improve how this land is used, the work I described in my Special Threats post around preventing Combined Sewage Overflows and strengthening both the infrastructure and preparation for flooding remains a priority. 

Mitchell, J. H. (2008). Paradise of All These Parts: A Natural History of Boston (1st ed.). Beacon Press. Weiskel, P. K., Barlow, L. K., & Smieszek, T. W. (2005). Water resources and the urban environment, lower Charles River Watershed, Massachusetts, 1630-2005. U.S. Department of the Interior, U.S. Geological Survey. Zarriello, P. J., & Barlow, L. K. (2002). Measured and simulated runoff to the lower Charles River, Massachusetts, October 1999-September 2000. U.S. Dept. of the Interior, U.S. Geological Survey. 

The fearsome Water Chestnut

The water Chestnut, or Trapa natans, is a invasive species to the northeast that first touched ground in the US within the Charles Watershed. Below you can find some information I have gathered on this species that overwhelms and smothers ponds, lakes, and streams beneath thick carpets of floating leaves.

Transparency Tube: Better than a Secchi disk?

Several years ago, I had experience as a volunteer and professional collecting water samples. As I started research for this post, I planned to focus on water sampling equipment and tests to make recommendations for an entry level water sampling kit. In that process however, I stumbled on a way to measure turbidity (how cloudy water is) that was new to me. In my days of water sampling, I always used a tool known as a Secchi disk but there is a cool tool called a transparency tube that is a better fit for many purposes.

Measuring the clarity of water is essential to understand how far light can penetrate for photosynthesis, animal visibility, and tracking the particles and nutrients in water.

Secchi Disk:

The Secchi disk is one of the simplest tools an enthusiast or scientist interested in water quality will use. It is made up of a rope attached to a weighted disk, usually colored white and black. To use a Secchi, you simply lower the disk into a body of water slowly until the disk is no longer visible, and then record the depth at which this happened by measuring the length of rope used when lowering. You can buy Secchi disks cheaply as scientific equipment goes, but they are also easy to construct if you want a project to start your first water sampling kit.

To show you how quick and easy taking a measurement with the Secchi Disk is, watch this video.

Video Source: AlaskaSeaGrant

Importantly, Secchi disks are best in bodies of water with deep, slow moving water because if the water is too shallow to reach a point where the disk is obscured or water is moving too quickly for the weighted disk to stay in place the Secchi will not give as accurate a reading of turbidity and water clarity. My experience water sampling was in a bay with slow moving water, both shallow and deep areas, and large variances in turbidity within a brackish estuary. In that environment the Secchi disk was perfect, and easy to train volunteers to use!

Transparency Tube:

The transparency tube is a tool that also measures the clarity of water and is relatively simple to use. In essence, a transparency tube is a transparent tube marked with measurement's along the side, with a valve as well as a miniature Secchi disk inside the base of the cylinder. More details can be found along with the diagram bellow at this link.

Image Source: J. Fetter and K. Koch, 2022

To use a transparency tube, you pour a water sample inside the tube and then slowly release water out the bottom valve until the mini Secchi disk becomes visible. You then record the turbidity according to the marks along the side of the tube at the point when the disk became visible. Transparency tubes normally use a measure called Nephelometric Turbidity Units or NTU. Importantly this scale is not linear and the area between 5 and 10 NTU will be much farther on a tube than 200 and 300 NTU for example. In contrast to Secchi disks, transparency tubes can be used in shallow water or swift flows.

Video Source: Washington Stormwater Center

If you purchase a transparency tube for your water sampling kit, be sure to store the tube with a covering that will prevent any scratches as the tube's transparency is required for accuracy. Likewise recorded the weather and light conditions when taking a measurement outdoors as those conditions can effect results. If the body of water you are interested in studying has slow moving, deep water I would recommend building your own Secchi disk before investing in a transparency tube for sheer simplicity, but these are a great option and provide accurate measurements of water clarity under conditions the Secchi performs poorly.