The Everglades have been called the River of Grass. Florida is barely above sea level--flatter than a pancake. But Florida gets substantial rainfall, so runoff during the winter rainy season flows in a broad sheet towards the southern tip. It's a river only inches deep, but many tens of miles wide. It supports unique ecosystems, and millions of birds.
Unaware of this natural system, engineers built roads and canals across the River of Grass, blocking it's flow. It began to dry up--the animals started to disappear. Recently (until the recession) Florida and the Feds pledged billions to restore the River of Grass, because it was so special.
We have something similar in Cape Cod--a River of Sand, or better, a river through sand. The rainfall sinks into the sandy soil, and flows slowly underground like a broad river, from areas of higher terrain and higher groundwater toward the coasts. These reservoirs, or sources of groundwater, are called "lenses." You may have heard of the Monomoy Lens, the one that's the reservoir for Harwich tap water.
There's an amazing chart that will tell you most of what you need to know about the Cape's groundwater flow.
There are other Rivers of Sand in North America--I'll introduce you to four others. But only on Cape Cod do we have so many "windows" into the groundwater. Those windows are the ponds. That's why Cape Cod--and its coastal plain ponds--are so special.
Two rivers of sand in Wisconsin
Southern Wisconsin (like Massachusetts) was covered with an ice sheet until about 12,000 years ago. At the ice cap's border near where I live in Madison, great braided chanels of meltwater flowed out from the melting ice. The floods enlarged their valleys, and partially buried them in sand washing out from the ice.
Today, the Wisconsin River is the largest in our state--but 12,000 years ago, it was much larger. The lower Wisconsin River valley, 93 miles long, contains about 4 cubic miles of sand. As a result, the river has become a favorite for paddlers and campers. Every camper can have their own private, sandy beach.
During the summer, the river shrinks substantially--but there's always ample water below the ground, in the river of sand. Because the soil is sandy and porous--it doesn't hold rainfall for long--so farmers pump the water from wells to irrigate with sprinklers.
Rivers of sand are prone to their own unique problems In Wisconsin, fertilizers and pesticides easily filter through the sand, to contaminate many wells or waterways. In Cape Cod, the analogous problem is the escape of nutrients (phosphorus) from septic systems.
In Wisconsin, too much pumping by farmers causes creeks and ponds to dry up.
In another river valley near Madison, we've got the opposite problem. At Fish and Crystal lakes, water levels have increased 9 ft since 1966. Hundreds of summer homes flanking the ponds are being flooded out. Most people blame it partly on increases in rainfall--which could be partly to blame.
What's the lesson for Cape Cod? Though a thousand miles apart, rivers of sand follow the same laws of physics.
As sea levels rise from global warming, groundwater levels on Cape Cod will respond. Some houses located close to the levels of ponds will be flooded out. Already, Hawksnest Camp is nearly flooded when the pond's level naturally rises each spring. In the long run, this cabin will be a casualty of climate change.
There's another warning for Cape Cod from Crystal and Fish lakes. They are badly polluted by runoff and seepage of nutrients from nearby farms. Cape Cod's river of sand is threatened by similar seepage from septic systems.
There's a reason why ponds on Cape Cod aren't as polluted as those in Wisconsin--Cape Cod never had the intensive farming that polluted most waterways in southern Wisconsin.
Two rivers of sand in Baja California
I've made several winter trips with my trailer to Mexico.
The Mexican state of Baja California Norte is extreme desert--and gets most of its rain from a few winter storms. Sometimes they are hurricanes. In some years, no rain falls at all. So when the deluge comes, bone dry valleys become torrents. Because of the occasional flooding, the valleys have become choked with sand, much like the Wisconsin River.
But while these valleys seem dry, there is still underneath a river through the sand. When I first saw this valley in the 1960s, there was a village of a hundred impoverished Mexicans not far from the sea. In our jeep, we passed a single burro heading down the dusty track towards the town.
Fifty years later, El Rosario has about 40,000 inhabitants, thanks to water pumped from the river of sand. They use it to irrigate the tomatoes and peppers that eventually reach your supermarket. But every few years the torrents still come to refresh the groundwater--and also to ravage the fields and wash out the highway over the river
Looking for a swim in the desert
On my last trip, I had a chance to explore a little river of sand in the high desert. It was a small creek, flowing out of rounded mountains of white granite near Catavina.* Upstream from the highway, there were some rocky pools ideal for a cool swim, but I kept getting interrupted by exploring tourists. So I decided to head downstream from the highway, to find my own private swimming hole.
Unfortunately, below the highway the canyon floor became sandy--and the feeble stream flowed only a few inches deep. Then, the water disappeared almost entirely--sinking into the river of sand. Disappointed, I climbed onto a boulder in the middle of the wet sand to look further downstream. Not seeing any pools, I jumped back to the wet sand, ready to retrace my steps.
But to my great surprise, upon landing I sank up to my thighs in quicksand. Damp sand that had looked solid--now became like jelly when I thudded onto it.
Here's what must have happened. When groundwater flowing through the sand met the boulder in its path, the water was forced upward through the sand grains. If the water rises fast enough, it can push the sand grains apart, so they no longer touch and support one another. Then the sand becomes like a heavy fluid--quicksand. If the sand contains just the right amount of water--if it's close to becoming quicksand--a sudden shock (like my landing) can turn it from solid to liquid.
This change sometimes happens during earthquakes. Large areas of wet floodplains can suddenly be turned to jelly, carrying away buildings that once stood firmly on top. In an instant, a solid river of sand can really become a flowing river.
But not on Cape Cod. No lesson here--just an adventure.
A pond is a window
This look around the world shows how special Cape Cod ponds really are. They are windows into the groundwater. They allow a glimpse into what is otherwise unseen, but so important to the Cape. The river under their feet is where Cape Codders, who are surrounded by salt, get all their fresh drinking water.
The ponds used to be incredibly pure. All the groundwater on the Cape came from pure rain, and if any that fell got polluted by a town, it was filtered by cubic miles of sand. When I was a boy swimming at Hawksnest, I used to drink the water as I swam.
With hardly any agriculture, Cape Cod avoided the first big threat to water quality. But then came the rush to build summer and retirement homes. Individual septic systems multiplied. Even when they function perfectly, septic systems still release nutrients into the groundwater. So all Cape Cod ponds are under threat from increased nutrients, which sometimes contribute to toxic algae blooms.
The degree of threat depends on where the ponds are situated along the river of sand, and on the amount of surface runoff (from lawns and pavement), and on erosion. Already, a number of ponds on Cape Cod have suffered toxic algae blooms. At Cliff Pond, at least three dogs died within minutes of drinking the water.
Because Hawksnest is located near the center of the Monomy Groundwater Lens, it's like a mountain lake, protected from polluted inflow. The land around it is protected from development, and the ring of shore vegetation is remarkably intact. So Hawksnest stands an excellent chance of staying pristine, if it is managed properly.
Some of the ponds on Cape Cod are still amazingly pure, compared to most other waterways around the world. They are warm, crystal clear, with sandy bottoms, and fragrant to swim in. Flowering shrubs surround them--as you swim, fragrance descends to the surface, then spreads across the mirror surface of the pond.
Rare or elusive plants and animals live along the shores of Hawsknest--endangered damselflies, delicate sundew plants that eat insects, lovely Plymouth gentian flowers, or secretive otter that play in the mists of dawn.
Together, these make up the rare** ecosystem of coastal plain ponds. These are quiet places, sheltered from the winds and tides that ravish ocean beaches. The plants have adapted to this inland calm--they ring and protect the gentle waters, holding the pond close with their twining roots.
For a while, the pond rises, pruning back the small plants and seedlings on the beach. But as summer advances, or sometimes in years of greater drought, the sandy beach emerges for a while.
Then the smaller plants have their chance. The tiny sundews come from nowhere--then spread their sticky fingers to ensnare blundering gnats. Plymouth gentians sprout from long dormant seeds or rhizome--to blaze again in tall sprays of purple, daisy-like flowers.
This slow, back and forth dance of the waters--is what the ponds are all about. This ring of fragrant shrubs is not something that can endure much trampling. It's a gentle place. It asks for gentle visitors.
Vehicles and campfires are for the ocean beach, where winds and currents surge. Salty shores are used to disturbance. Ponds are all about a gentle ebb and flow, watched over by a delicate ring of vegetation that protects the pond.
Common but rare
There are other rivers of sand. But in southern Wisconsin, ponds are fewer and almost everywhere polluted by agriculture. In Baja, the rivers of sand sometimes become raging torrents--and there are no ponds at all--only a few rocky pools in desert canyons.
So pure water for swimming,** and intact shoreline vegetation, is quite rare around the world. The good news is, that in Cape Cod, there are still a number of pristine "windows on the groundwater" left.
The bad news is, that because there are so many rare ponds in one place, people don't appreciate this treasure in their own back yard. Through neglect, it could be slipping away.
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* The canyon is named "Arroyo la Bocana" further downstream.** Is "pure water for swimming" really that rare? What about lakes in the mountains, or the north country?
There is still a lot of relatively pure lakes and streams on this planet. But it's usually too far away for your vacation. Mountain lakes or northern lakes are often too cold to swim in. In Northern Wisconsin, our lakes are pretty clear, but they are cold, dark with tannin, have mucky bottoms, and are guarded by swarms of leaches and mosquitoes. Before you can swim in mountain lakes, you often have to wade over sharp or slippery boulders.
I visited four mountain lakes this summer in the Cascade Mountains of Oregon. Two were as clear as Hawksnest (but colder), one was clear but had the shore destroyed by hikers and horses, and the fourth (a large crater lake with hundreds of campgrounds) was somewhat polluted. Not one was as convenient or pleasant for swimming as Hawksnest. In all of the lakes, I saw only one person swimming, besides myself.
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