▁can ▁be ▁ha z ard ous ▁due ▁to ▁we ed ▁growth , ▁which ▁forms ▁a ▁dense ▁blank et ▁in ▁some ▁channels , ▁but ▁a ▁d rain ▁that ▁was ▁formerly ▁virt ually ▁un - n avig able ▁may ▁suddenly ▁be ▁cleared ▁to ▁improve ▁its ▁d rain age ▁performance ▁and ▁become ▁navig able ▁again . ▁One ▁unexpected ▁possibility ▁of ▁cru ising ▁the ▁dra ins ▁is ▁to ▁visit ▁New ▁York , ▁a ▁ham let ▁just ▁to ▁the ▁north ▁of ▁H ough ▁Bridge ▁on ▁the ▁West ▁Fen ▁D rain . ▁ ▁Struct ures ▁ ▁The ▁relative ▁isol ation ▁of ▁much ▁of ▁the ▁With am ▁N avig able ▁Dra ins ▁has ▁resulted ▁in ▁many ▁of ▁the ▁structures ▁which ▁were ▁built ▁as ▁part ▁of ▁R enn ie ' s ▁upgrade ▁in ▁the ▁early ▁ 1 8 0 0 s ▁surv iving . ▁Because ▁of ▁their ▁historic ▁interest , ▁they ▁have ▁been ▁added ▁to ▁the ▁listed ▁structures ▁register , ▁and ▁are ▁Gra de ▁II ▁List ed . ▁They ▁include ▁a ▁series ▁of ▁red - br ick ▁brid ges ▁with ▁g rit stone ▁cop ing . ▁On ▁the ▁Hob hole ▁D rain , ▁Fre ist on ▁Bridge ▁has ▁three ▁elli pt ical ▁arch es , ▁while ▁In gs ▁Bridge , ▁H ods ons ▁Bridge ▁( B ridge ▁No . ▁ 8 ), ▁Bridge ▁No . ▁ 9 , ▁and ▁Station ▁Bridge ▁( B ridge ▁No . ▁ 1 0 ) ▁have ▁a ▁single ▁arch . ▁Hob hole ▁S lu ice , ▁which ▁is ▁constructed ▁of ▁g rit stone ▁as hl ar